Deluge
I'm currently running deluge 1.3.3-dev with libtorrent 0.15.6.0 on a debian linux box.
Just wanted to warn any of you who are running on linxu with btrfs, don't upgrade to kernel 3.0.0. I have actually confirmed this problem exists for deluge 1.3.2, deluge 1.3.3, deluge 1.3.9, and azureus/vuze. Bittorrent downloading to a btrfs filesystem will cause very high IO rates that eventually hang the bittorrent program. Downgrading to kernel 2.6.38-2-686 completely fixed the problem.
Btrfs is really cool, but I guess they're serious about the fact that it isn't ready for primetime. This is the first problem I've had with it in several months of using it. I've submitted a debian bug report (Bug#638467) in the hopes that I might be able to help find the real problem.