Wow, it's been four years now since Sun launched OpenSolaris. We've come a long way since then - built up a budding community, taken lots of contributions from outside, and we're even turning out a pretty decent OS based on this now! It's on my desktop, laptop and home machine. There's still a lot to do, but overall I'm very impressed.
It's been very cool doing code reviews openly and getting design feedback directly from the real world before any code is even written. This has greatly changed the way I do my job, for the better!