For a while, I've been assembling a giant collaborative google doc of hackathon/startup project ideas with my friends that's been quite successful: http://hackathonprojects.tk/ Notably, we've developed a syntax for linking ideas to each other, so that people browsing the site can see hard-to-see relationships between people and projects that others have found. I've also been working on a dedicated web platform for this purpose (to facilitate ideation): IdeaOverflow. When you start typing in an idea, it uses MIT ConceptNet 4 as well as the manually established graph of ideas already present to attempt to suggest related ideas in the database.

I have larger aspirations as well... http://ideaoverflowideplan.tk/

Email me at [email protected] if you'd like to work on this with me by the way! Basic django (or php! or js frontend!) skills are sufficient to be useful, though making a good interface to explore graphs is also a high priority... http://arborjs.org/ is almost right but not quite.