- Alfa Networks AWUS036H: Very, very good... but a little expensive
(at least here in Brazil). The best driver for this adapter is the one with the Backtrack distribution (www.remote-exploit.org). Since it is used a lot for Wardriving, etc.. monitor and ad-hoc mode is very stable. USB.
Haven't tried the driver which is in Backtrack yet. But the current one shipped with Debian unstable does not allow ad-hoc mode, just managed and monitor mode (with packet injection possibilities).
Cheers, Linus