Rails is moving to Git (and Lighthouse)
The Rails core team is making a big move to Git (and Github) for source control and Lighthouse for issue tracking. I love the move to github - it will make managing patches and your small tweaks to Rails much easier to keep in sync with your chosen branch of Rails. The move to Lighthouse I'm not sold on, only because I haven't felt the big benefits it offers over Trac, though I haven't really used Lighthouse much over the past few months.
We've been moving all internal things at Relevance to Git and Github, and so far we've been very happy with the move. Local commits are great, git is blazing fast, and github is the bees knees. If you haven't learned Git yet, now is the time.