Best Buy offers the Belkin N600 Dual Band 802.11n Wireless USB Adapter, model no. F9L1101, for $15.99 with free shipping. That's $4 under our March mention and the lowest total price we've seen. (It's the best deal we could find today by $4.) It supports speeds of up to 300 Mbps.
Micro Center offers the Belkin N150 802.11n Wireless USB Adapter, model no. F9L1001, for $4.99. Opt for in-store pickup to dodge the $5.99 shipping fee. That's the lowest price we could find by $2 and the best no-rebate price we've seen. (We saw it for a buck less after rebate three weeks ago.) It features up to a 150Mbps data rate.