I produce a monthly summary of the main platforms / dApps with their user, post, and payout numbers. You can find the most recent version here:
Probably just focusing on the top 25 will give you a good idea of the main platforms:
steemit
busy
esteem
dtube
dlive
steepshot
steemhunt
tasteem
dsound
steempress
steemauto
dmania
steemjs
steemkr
hede
steemeasy
partiko
bescouted
zappl
sprtshub
piston-lib
ulogs
elegance
steempeak
comus.fun
And add musing.io obviously!
Some of the Apps below the top 25 are still in development (something I will separate out in future). Some (like steemjs and piston-lib) are actually developer code libraries for accessing the blockchain rather than particular apps.
Hope that helps!