subversion

If your currently using cvs, I would strongly suggest considering subversion as a replacement. It has atomic changes, and deals with checkins as a group. If one file changes the set is marked accordingly, so you don't need to tag. This is quite usefull in a build environment.

If your going to switch and are using windows, I would also like to suggest some gui's, theres rapidsvn (which I don't use that much) and tortoisesvn. Tortoise integrates with windows explorer, and provides a good bit of functionality. I would also strongly suggest the usage of the cvs interface of similar name tortoisecvs