Code Bubbles are
a novel user interface metaphor for code understanding and maintanence (sic) based on collections of lightweight, editable fragments called bubbles, which form concurrently visible working sets.
This project looks interesting, a little like VisualAge on steroids. Like Mylyn, it may be more useful for getting to grips with existing code than creating new code. In fact:
A quantiative user study indicates that Code Bubbles increased performance significantly for two controlled code understanding tasks.
The debugging support is neat: you can save the state of previous sessions for comparison, and separate output by user-defined prefix criteria. You can also ask it to show you the code path between two bubbles of code.
And it supports keyboard navigation!
For more details and a video, see http://www.cs.brown.edu/people/acb/codebubbles_site.htm