There are a lot of cheap no-name brand USB dongles that work in linux. They all seem to be based on the Atmel at76c505_rfmd2958 driver available here:
http://at76c503a.berlios.de/
There's also a sourceforge based driver here:
http://atmelwlandriver.sourceforge.net/news.html However it's not as well written and I've had trouble getting it to work.
If you can get a Netgear MA111 they are a prism chipset and are supported by the linux-wlan-ng driver (
http://www.linux-wlan.com/linux-wlan....html#Download) however the prism driver doesn't support the iwconfig interface so it's hard to configure.
Also adaptors like the Netgear WG111 (NOT WG111T) that are based on a prism54 chipset now work with the prism54.org driver.
In my experience it hasn't been too hard getting USB wifi's to work, and they don't tend to chew CPU or anything either.
edit: I regularly use a cheap Atmel based dongle that works fine, and I've recently got a MA111 (the only non-modded adaptor of mine

) that works fine as well. I've also tested a different brand (still some small makeshift company though) USB Atmel one that worked fine also.