Bitkeeper
Bitkeeper is a source-code control system that's distributed, concurrent, and lots of good things. It's no longer available free of charge for open source projects, so it is no longer used for Linux kernel development.
The main reason to use it was that Linus did --- if you want to be able to submit changes to him, generating them with bitkeeper makes it easier.
BitKeeper/Basics --- Basic BitKeeper concepts
BitKeeper/IA64 --- Using BitKeeper for IA64 development
BitKeeper/Tricks --- Tricks and Traps with BitKeeper
