Btrfs is in mainline now.
Linus pulled this excellent filesystem into the vanilla tree. So if you were curious about how to extend your storage, this is a good way.
But beware, that it is a development tree and very likely contains fair number of errors you may encounter and thus lost the data.
Contrary to this we can check how Andrew Morton opposes to merge SquashFS (compressed read-only filesystem) because of not enough of testing (it is a default FS to build live CDs and installators for several years in all distros), not enough review (it passed three in the fsdevel maillist, although my trivials were not commented :), something else and (!) because it is called after a vegetable. I do hope Andrew made a joke about the last argument, even Linus entered the discussion, and now SquashFS is effectively merged, since Andrew agreed to pull it into the tree.
I’m curious what will happen when I will push POHMELFS upstream (and I will do it soon enough). Actually while I was writing this I decided to push it just tomorrow after documentation update. At least to ask Linus and Andrew, and you know, I can bet you there will be zero responses (related to the process), although POHMELFS is very competive already (although a bit younger than btrfs).
And although I’m pretty sure about the result, let’s do this for fun :)