Mercurial
I started using Mercurial at a client in 2009. We checked into Mercurial and then synced into ClearCase from there (one-way). Obviously, this only works if everyone goes through Mercurial -- there will be problems if someone independently checks changes directly into ClearCase.
It would have been best to install this on some UNIX box. However, very few of the UNIX boxes around here have C compilers, and even fewer have Python, so we're out of luck. That meant running it on my PC. We wanted to use ssh as the access method, so that meant installing OpenSSH -- and the only viable option for that is Cygwin. This opened up a whole other can of worms. (Eventually I did end up finding both Solaris and AIX boxes that had compilers, and we pushed to the UNIX boxes instead of my machine.)