mwave offers the refurbished EnGenius 802.11n Wireless USB 2.0 Adapter, model no. EUB9603EXT, for $11.99 with free shipping. That's $18 under the lowest total price we could find for a new one. It features a 2dBi detachable omni-directional antenna, a maximum data transfer rate of 150Mbps, 64-bit/128-bit WEP/WPA/WPA2 support, and more.

Of note, a 30-day warranty applies, although it's unclear who supports it.