linux-next

How to develop against linux-next
http://lwn.net/Articles/289245/

Now set up remote tracking branches (I tend to do one for linux-next and one for linus as):

git remote add -t master -f linus git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git
git remote add -t master -f next git://git.kernel.org/pub/scm/linux/kernel/git/sfr/linux-next.git

The point of keeping the linus tree is that eventually all the branches that comprise next are merged into linus, at which point your patches will be ready for submission against vanilla linus.
To download the current trees of the day, simply do a

git remote update

[wiki] Linux kernel
http://en.wikipedia.org/wiki/Linux_kernel

' Linux Next ' Begins To Take Shape
http://www.internetnews.com/skerner/2008/02/-linux-next-begins-to-take-sha.html