To avoid derailing my own thread, I thought I'd ask what IF platforms people know of, or even better, have tried and can report on. Here's my list:
CYS: We all know this one, of course. Text followed by choices, with limited variables and a reasonable amount of scriptability. Notable features include a web-based UI, the ability to collaborate with other users, and a built-in inventory system.
Twine: Text with links wherever you want. Variables can be text, including user-entered text. Highly scriptable. Notable features include a node-based GUI that lets you visually map your work, and full customizability of the look of your final piece -- if you know CSS.
ChoiceScript: Text followed by choices. Variables can be text, including user-entered text. Scriptable -- I'm not yet sure to what degree. Notable features include the ability to be commercially published by Choice of Games, with distribution to a variety of platforms.
Three more in the 'text followed by choices' vein, albeit with prettified interfaces, none of which I can really comment on in depth: inklewriter, Varytale, and Undum. Anyone here used any of these?
And then some options that are a bit further afield:
StoryNexus, based on virtual cards.
Ren'Py, for visual novels.
Inform 7, for parser-based IF.
Again, I've never used any of these to any real extent, but I'd be curious to hear from anyone who has! And, of course, anything I've missed entirely.