Wireless

Can be a little tough as few manufacturers bother to supply drivers, but the Linux boys have compiled lists of the chipsets used in most adapters and you can probably find a Linux driver for most chipsets.

 
This is my favorite Linux USB adapter.

Some of the best supported Linux chipsets are:
RT73 Ralink
RTL8187 Realtek
Most Athereos are supported as well

Steer clear of Broadcom, as they're hit and miss depending on the model.
Ralink are widely supprted, but a few are not.