As part of my studies, I am currently working in a three week internship at university, consisting of 4 tutors, 6 fellow students and me.
We are improving on the Continuous Quality Assessment Toolkit; or short: ConQAT.
It's a nice little open-source code quality analyzer that works as an Eclipse plugin. It comes with HTML result generation, a very flexible pipe-like mechanism to build analyses out of simple parts (called processors) and a graphical editor to build those.
Most of our work is focused on the GUI part of the application, since the rest is already pretty rock-solid.
It's quite a lot of fun and the insight you gain into the Eclipse architecture is quite... enlightening ;-)
As of late, I'm working on refactorings, quite an interesting, challenging (and not at all well-documented) part of Eclipse.
But I'm doing well so far. Quite the learning experience.