Mwave offers the PowerLink Hermes 300Mbps 802.11n Wireless USB 2.0 Adapter with Dual 9dBi Dipole Antennas, model no. PL-18N, for $39.99. Coupon code "MLCP05EN" cuts it to $24.99. With free shipping, that's the lowest total price we've seen and a current low by $4. It features a max speed of 300Mbps, operating distance of up to 1.2 miles, purported 10x signal strength and 1000mW power output, dual 9dBi dipole antennas, and support for WEP, WPA2, and WPA-PSK encryption.

Update: The coupon code has expired; however, the price has dropped to $24.99.