Command line clients
Of course, the standard CVS client is available everywhere. I believe http://www.cvshome.org/
is the current home of CVS, and you can download the source for the client and server from there. The current version seems to be 1.11.1.
Graphical clients
This is an incomplete list. A pretty complete list is available at http://www.cvshome.org/dev/addons.html
.