The Alfa AWUS1900 is a nice model, but if you want something cheaper (and will put up with 2.4ghz-only), the TP-Link TL-WN722N is cheap, but get the v1 chipset!

The Alfa AWUS1900 is a nice model, but if you want something cheaper (and will put up with 2.4ghz-only), the TP-Link TL-WN722N is cheap, but get the v1 chipset! It's the best-supported with drivers on linux. Oh yeah, and you will probably need to do monitor mode on linux.