If you want to know what's going on with Open Solaris below are some useful links to start-up with. As a lot of features currently in development will be backported to Solaris 10 it's really informative to sys admins to look around so you can know what's coming and what to expect in Solaris (and all the rest features will go to Solaris 11).
If you want to know something more about Fire Engine, Nemo, and other networking staff visit OpenSolaris Community: Networking.
For developers and others interested in how ON (and what is ON) is being developed OpenSolaris Community: Nevada is a place to go.
And you can get list of all the other Open Solaris Communities here.
You can't miss Bug Database - I found many bugs I issued :)
Source Browser - this one is really great! It's really helpful not only for developers but for sys admins too. Now if you get some "strange" log or command output you can just try to search for in sources - it can help you to get answers quick & easy. It's a good fun too :)
If you want to subscribe to mailing lists (I strongly encourage you - there're many talented people from Sun and from community already) go here. Most general mailing list is Open Solaris Discuss. It's a good start.
Now, if you want to get Open Solaris sources and compile them you should go here. It's really simple to compile your own kernel - just follow instructions. Even if you are not a developer it's a lot of fun to do so. And if you got in trouble - just post your questions to Open Solaris Discuss list mentioned earlier - I'm sure someone will help you.
Some people asked me how easy it's actually to compile your own kernel which is bootable. Well, it's really easy! Once you get sources & tools minimum is to setup a few enviromental variables then type 'nightly ./opensolaris.sh' - and you should get kernel (full ON actually) compiled! Yes, that's it! Then you can BFU your system (just one command), resolve conflicts (manually or using again just one command) and reboot your server - now it'll boot your own compiled kernel! So again - just in a three commands you can compile and boot your own kernel! (actually four commands if you count 'reboot'). For detailed documentation look at Release Notes (should be enough for just a try) or if you really want to know what you are doing read even more detailed guide here.