I thought I'd contribute and share this collection, most of these mods I am either in the process of adding or already have though there are some I wont use but will still link here for those who might use them, feel free to add anything cool that I may have missed!
You will notice that there are lots of player homes here, that's because they seem to be the most common kind of solstheim mod, included in the list are a few highly popular mods as well as a few unknown little gems that are decent additions to the game.
NEW DUNGEONS AND CAVES
Solstheim - The Lost Levels - Although this mod has had some complaints I love it, it adds 6 new and unique caves, a few new plants and four new quests to the game, it fit in well with the world and I only had a minor texture issue witch resolved itself, great mod.
Solstheim Dungeon Pack - This adds 2 new ancestral tombs, one new cave, one huge new temple, a sewer in raven rock and a new nordic ruin as well as a morag tong hideout, though the comments say this is not a hugely action based mod it's still very well received and apparently well designed, will be checking this one out. (uncertain of compatibility with the lost levels but I will report back)(EDIT: This mod is phenomenal and deserves more attention)
NEW PLAYERHOMES
Ashborn Farm - adds a small player home/farm near Kolbjorn barrow guarded by a pair of ash spawn. (EDIT: Did get a little it of floating props from this but it was likely a mod conflict, my guess is the Roads of Solstheim mod which admits to modifying the land)
Severin Manor Improvements - a popular mod which fixes up the player house in solstheim, its a pretty cool mod and there is a full list of additions and features on the mod page so check it out.
Severin Manor Guard - adds a housecarl to severin manor once you talk to captain valeth, if loaded before the above mod the two work fine togeather.
Archagruhn - this adds a player home stronghold with its own plane of oblivion, located in the ashlands to the south.
The Field Lab 2 - This mod adds a small courier initiated quest to obtain a special spell and new player home, it's telvanni themed and I believe to complete the house you do need to go to solstheim though I am not sure, still it's a great addition to take to solstheim.
Frost Ivy Glade - By all accounts its a very good house mod, located just a bit out from Skaal village
Bayview Manor - adds a new playerhome above skaal village, haven't installed this so I don't know if it clashes with the other skaal village mods but it would be cool if it didn't.
EXPANDED SETTLEMENTS
Skaal Village Extended - adds a new player house among other things and makes skaal village feel better.
Tel Mithryn Extended - a great little mod that expands tel mithryn with a little player home and some misc clutter dotted around, looks very nice. (EDIT: Tested and works alongside Tel Mithryn House making an even better settlement)
Raven Rock Extended - Best mod on the list, it is without doubt the best Raven Rock Overhaul and I would not play without it, it adds a new bookstore (with new books) a new clothes store, a fishery, a magic shop, western side of raven rock fortifications, more morrowind styled banners, more clutter, a new set of patrolling guards who head out further and another ashlander camp near tel mithryn. The ashlanders make their way to raven rock every other day to trade with the people, rest and pray, there are also two skaal hunters who visit, great mod, highly recommend.
POPULATION/NPC MODS
Populated Solstheim - This is a nice and inconsequential mod that adds a few immersive npcs wandering around doing their thing, I find they make raven rock feel more alive and think it's essensial for solstheim.
More Solstheim Followers - Adds two new dunmer followers to the retching netch in raven rock and two new nord followers to the great hall in skaal village, good well thought out companions that work well with most companion mods.
Morrowind Immersion Project - nice little mod that adds a bunch of cool changes including cliffraces and new cliffracer items, a new telvanni settlement, guars in raven rock, a plain strider mount, a scamp merchant and a group of telvanni guards at their respective settlements.
Vijia in Solstheim - not entirely sure what this entails but it's listed as more immersive solstheim experience with the Vijia companion, pretty essensial if you have the original mod.
NEW ITEMS
Dungeon Treasure - Dwemer and Ancient Nord coins - This is a fantastic mod in my opinion, instead of finding piles of imperial septims locked away in ancient pre-empire untouched ruins you now find coins that represent their ruin, so in a dwemer ruin gold is replaced with slightly more scarce but higher valued dwemer coins and the same with draugur ruins and ancient nord coins, great for immersion and adds another reason to delve into those dungeons.
Useable Ash Spawn Weponry - Somewhat of a hidden gem in my opinion, this mod lets you take those axes the ash spawn sometimes use and it also adds the ability to improve them at the grindstone with spawn ash, its been a great and seamless edition to solstheim for me.
Bonemold Expanded - adds bonemold and chitin armor of each dunmer great house and distributes them around solstheim.
Bonemold and Chitin Weapons - Morrowind Armory - This mod adds 18 chitin and bonemold weapons to the game, spread throughout both skyrim and solstheim's levelled lists, great if you want more variation, there is also a patch for the pit fighters mod which makes the pit fighter mod added chitin weapons conform to this mods design (pretty nifty)
TEXTURE MODS
Ash Rocks - This mod replaces the gray paint-like mess that is splattered on top of many rocks and mountains of Solstheim.
Detailing the Eldrich - Higher-Res Apocrypha - This will replace the textures for the realm of Apocrypha, the Exterior of Miraak's Temple, the Arches around the Sacred Stones, and the Black Books. All files have been worked up from the official high-res pack, most textures have been reworked from scratch, using the originals as reference. The new textures are mostly 2048, while some, mostly normal maps, remain their original resolution, the original alpha channel info should be intact across the board.
Detailing the Eldrich - Higher-Res Riekling Architecture - This will replace the textures for Riekling tents, huts, stands, clutter, and brigdes. All files have been worked up from the official high-res pack, most textures have been reworked from scratch, using the originals as reference. The new textures are mostly 2048, while some, mostly normal maps, remain their original resolution, alpha channel info should be intact across the board.
Solstheim landscape and furniture - Almost full replacement of textures for a landscape Solstheim. Except those textures which are used from original game, certainly. I tried to make them similar on original, but lighter, colourful and cheerful.
Skyrim Realistic Texture Overhaul Dragonborn - another re-texture for dragonborn.
Solstheim Landscape Overhaul - Solstheim Landscape Overhaul provides two different editions: fertile and dead. This mod alters the look of Solstheim with new trees, grasses, textures and other assets.
aMidianBorn Solstheim Landscape - texture replacer for Solstheim landscape set , ground rocks moss, NO architecture.
SMALLER INCONSEQUENTIAL/MISC MODS
a Quality road map - Solstheim - if you've searched Solstheim on the nexus then you have likely seen this mod before, its the most endorsed mod with solstheim in its name, a great mod that replaces the map to be more aesthetically pleasing and actually shows you where the roads are unlike the vanilla map.
Wet and Cold - Ashes - This mod makes npcs react to solstheim weather better, npcs will protect themselves from dust storms and the player will even notice the effects of the weather conditions, great and immersive mod.
Wolves of Solstheim - This mod increases the quantity of wolves bears and werewolves in solstheim, the only issue I have seen with this mod is the lack of prey for such a large quantity of predatory animals to exist, so may not suit some.
Ash Magic - Adds 13 new ash based spells for all you telvanni mages out there, including the ability to summon bound ash weapons and ash spawn.
Dragonborn - Solstheim Boat Travel - - This mod adds a boat at raven rocks docks allowing for fast travel to tel-mithryn, haknir's shoal and northshore landing, it costs 150 gold and aside from one CTD report everyone seems to love the mod.
More ash on the roads - This mod changes the roads to make them more ash covered, don't have it myself but it looks pretty nice.
Roads of Solstheim - Made by the same modder as 'more ash on the roads' this mod replaces the black marks as roads with actual paved roads like in raven rock.
I'm sure there are a few I'm missing but hopefully some of the above will suit you!
EDIT: ADDITIONS SINCE ORIGINAL POST
TEG Solstheim - created by the now infamous giskard, still an interesting looking mod that might suit some of you, it adds the temple, the morag tong as a guild and the ability to join the redoran guard and defend against attacks, adding many new quests, radiant and otherwise. There is no nexus page just a direct download link, here's a mod description from giskard.
Indarys Manor - Redoran themed mansion - this adds a new worldspace for a new player home with a hireable guard and staff, there is also a village and a kwama mine (morrowind nostalgia) and a durzog pet, this is an incredible mod and the new worldspace is a welcome edition to the solthsteim experience.
Old Dusty Travels again - Another hidden gem that was pointed out to me, this mod adds a functionality to the last tame siltstrider on solstheim, another cool variant to fast travelling that goes well with the solstheim boat travel mod to add a series of ways to get around.
Telvanni Bug Musk Merchant - Adds a dunmer merchant who happens to be selling some of what may be the last Telvanni Bug Musk, along with some other Morrowind classics. He is located near Tel Mithryn, Telvanni Bug Musk allows you to haggle for 35% better prices and increases speechcraft by 20 for a duration of 90 seconds. It also comes in a pretty bottle.
