bitkeeper

BitKeeper is not an open source version control system, but is listed here for completeness because some open source projects use it. BitKeeper is very reliable and feature-rich, supporting distributed repositories; serving over HTTP, file, and directory copies, and renames; patches management; tracking changes from branch to trunk; and many other features. BitKeeper comes in two licenses. The commercial license costs a few thousands dollars per seat (lease or buy). The gratis license is available for development of open source software, but has some restrictions, among them a non-compete clause and a requirement to upgrade the system as new versions come out, even if they have a different license. Furthermore, the source code is not publicly available, and binaries exist only for the most common systems, including Win32. A handful of projects use BitKeeper, including some of the Linux kernel developers and the core MySQL developers. It has been the subject of much controversy in the Linux Kernel Mailing List. Due to its license, BitKeeper is not suitable for open source development, as this will alienate more "idealistic" developers, and impose various problems on the users who choose to use it. If you are working on a non-public project and can afford to pay for BitKeeper, it is naturally an option.