BitKeeper is the VersionControlSystem used for the LinuxKernel development since sometime around 2.4.13. Its major distinguishing characteristic is that it keeps distributed repositories, unlike the CVS model where everyone commits to the same machine.

It is not FreeSoftware that does some strange things like making all your changes/log files public unless you buy a commercial licence. (The idea is that OpenSource projects can use it without charge, but people who want to keep changes proprietary need to pay up.) I'm not sure about the licence details, but I think it requires you to accept changes to the licence, and to use newer versions of the software.

Another contentious issue is that they changed their licence so that people working on a competing product, eg SubVersion, cannot use it. There has been a lot of anger over this move, probably rightly.