Newegg offers the refurbished TP-Link 802.11n N300 High Gain Wireless USB 2.0 Adapter, model no. TL-WN822N, for $6.99 with free shipping. That's $3 under our Black Friday weekend mention of a new unit and the lowest total price we've ever seen for it in any condition. (It's also at least $13 less than you'd pay for a new one.) It features dual 3dBi omni-directional antennas, a maximum data transfer rate of 300Mbps, and 64-bit/128-bit/152-bit WEP/WPA/WPA2/PSK support.
A 90-day warranty applies, but it's unclear who backs it.