by far the most active is gridcosm (http://www.sito.org/synergy/gridcosm/). it has a fairly rigid structure -- you add one (or more) of 8 pieces around a central piece. its easier to just look at than explain!

the loosest active synergy project is PANIC (http://www.sito.org/synergy/panic/). this is basically a place to trade images. there is a bit of structure to the file-naming convention, and some web interface to facilitate the exchange, but that is about it. usually people email each other, chat on irc, or let people know stuff got put there in our discussion forums. the activity there (as the dates on the files contest to) can be erratic at best... its a free-for-all, and often just a fun place, even if in slow-motion. (you can, of course, work on files you see there and send back your results; or start a new file of your own.)

hope this gets you on the right track.