Git

Git is a free and open source distributed version control system designed to handle everything from small to very large projects with speed and efficiency.
Git is lightning fast and has a huge ecosystem of GUIs, hosting services, and command-line tools.
Git's performance and ecosystem
Pro Git book, videos, tutorials, and cheat sheet
Command line tools, GUIs, and hosting services
Git's reference documentation
Binary releases for all major platforms.
Get involved! Bug reporting, mailing list, chat, development and more.
Pro Git by Scott Chacon and Ben Straub is available to read online for free. Dead tree versions are available on Amazon.com.