I wrote a plugin for Hudson today, which integrates Devel::Cover (Perl's test coverage tool) into the build reports.
Actually, that's currently an exaggeration. All it does is add a checkbox in the configure page, and a link to Devel::Cover's reports on the build page when it's enabled. I spent the day remembering how to program in Java.
Tomorrow I might be in a position to extend it into something more attractive - I'll publish it very soon, but I need to run it past my employer. Watch this space.
In other news, I volunteered to package Hudson for Debian, and then discovered just how many dependencies it has. This should keep me going until squeeze+1, I think.