Btrfs source repositories
Btrfs has switched to git for version control. So far only the unstable btrfs repository has been converted from Mercurial, but the stable repository will be converted soon as well. The unstable repository includes the full kernel sources, and usually requires the latest mainline kernels.
Stable Repositories (still in Mercurial)
These hold code that is generally expected to work, and they are updated for major releases or critical bug fixes. Disk format changes will be announced at release time.
To download the stable kernel module and utilities:
hg clone http://www.kernel.org/hg/btrfs/progs
These hold code that is under heavy development, and they can be expected to be very buggy. The disk format may change in the unstable repositories without warning, and they should only be used if you are an active Btrfs developer.
To download the unstable kernel module and utilities:
There are also some ebuilds for gentoo!