UpRightDown is a project that creates multiple versions of the same basic plot scenario. The first issue was pretty simple; this issue, though, there’s a multi-stage episodic plot that contributors can present as narrative, song, video, photo montage, game, or pretty much anything else. There are also prizes this time around for the best plot contribution ($300) and best performance of a plot episode ($1000).
Sound a bit complex? They do a more thorough job of explaining. But I know from experience that they’re open to IF implementations of plot ideas.
Emily Short's Interactive Storytelling 
I’m so glad you’re promoting this again! It was through your blog that I first found out about it and submitted my own piece, and I just submitted a “performance” for episode two. I’m sure I’m not the only one who hopes you might contribute again this time.
I second Brooks’s appeal. FUGUE, Emily Short’s IF, was the most viewed page in issue # 1 of URD.