It seems every project worth its name is racing towards having some sort of dvcs (distributed version control system) . To understand the difference between now & where they want to go is to know where they stand at the present. Most of the old school SCM (Source Control Mechanisms) were of the time where contributors were living nearby & the contributors interested in the project were not many. Also issues of control were of paramount importance.
Today however, the situation has changed, more & more people are contributing to FLOSS projects & many of them might not have the best of bandwidth available to them most of them. This is where dvcs come in, one can checkout the whole tree & do their playing around for extended times off-line. Whenever they connect they can send their changes to everyone who is subscribed to their branch as well as reconcile their stuff with everyone else's . So if 2-3 people have an idea & want to run with it can do in this new era, rather in the old way. Also all of them are pretty fast.
Ok. now for the tools, there are a whole bunch of them, bzr, mercurial, git, svk, monotone are the ones in the know & perhaps a few more which I'm not aware of.
As far as projects are concerned, they are also moving but the one which I'll be glad when it moves to using one of these tools is mozilla,
Just a small list of projects either using or at planning stage to use these dvcs (the list is bound to grow) :-
3. Mozilla Firefox
5. Linux Kernel
and so on & so forth. The list is by far in-complete & there are many a projects which are moving to one or the other dvcs tool . Its the workflow which will tell them which tool to use. The whole point, people are moving to a better way of doing things, so be there or be square ;)