Sun has been touting about releasing an open source version of Solaris for ages, and it finally appears the ball is starting to roll. The OpenSolaris page is now available for people to start visiting, but unfortunately the source code for OpenSolaris 10 isn't available for download just yet (due in Q2/2005). However, they have made the code for DTrace available. Simply put, DTrace is a cool program that allows someone to pick apart the inner workings of Solaris, or an application running on Solaris, all in real-time without the need for a specially compiled kernel - it used the bog standard Solaris 10 kernel. More info can be found on the DTrace page on BigAdmin.
I think the best analogy as to what DTrace does is summed up by Alan DuBoff, one of Sun's x86 kernel developers. He was quoted (not 100% correctly though) in a recent article posted on http://news.com.com for saying:
DTrace is like being in a nudist colony, there are no secrets - not even for Solaris.
How's that for an analogy for an highly technical program. 8-)