If you are interested in trying out interactive writing (either parser or choice based) but you don't know where to begin, then this would be a good starting point. Personally, I would add ChoiceScript to the list as well, even though it isn't open source (but neither is Inform7).
ChoiceOfGames publishing model is almost perfect as it gives everyone the necessary tools to produce high quality choice-based IF and a range of publishing options from fully commercial under their flag, down to do-with-it-what-you-will and deal with monetization and promotion yourself. I might write a full article down the line what a fantastic model this is for game and content creators. I might not agree entirely with their editorial policies for their flagship label but hey it's their own thing... but more on that later, perhaps.
