I've fallen in love in Ohloh. Not only makes it a cool " coding experience" bullet in your resume, but also gives you interesting information about projects. Like the most active contributors, individual file licenses or people with similar software stacks to yours.

Just lately I've found out a new, cool feature was added. You can compare projects in terms of codebase. Obviously it doesn't make sense for any 2 random projects but sometimes it reveals interesting information. Like this Ant vs. Maven2 duel.
It shows (at least to me, please comment) that each tool is in different phase of development. In terms of codebase, activity and number of contributors as well. I'm not sure if the 200+ vs.140+ stack counts reflect a real world usage share though. We'll probably see more reliable results once the number of people joining the site grows.
Too bad one cannot compare Netbeans and Eclipse codebase the same way. The different philosophy behind the projects is reflected even here at ohloh. While NetBeans has one entry, Eclipse is split into many ohloh project definitions.