opensolaris.org is now up and running, with the DTrace source available to the world. There's lots more to come, and it is really exciting we're starting with DTrace. Bryan already has a nice introduction written, and Adam has also put up a some interesting tidbits.
I've seen a few questions raised about the delay in releasing the full, buildable version. Mike Kupfer gives an inkling of what's still to be done. But, I'm sure that those who want to grunge about in the most cobweb-covered it-came-from-New-Jersey parts of the system will get their chance too. For now, there's already plenty of interesting code to wander through.