Best Buy offers the Roku Streaming Stick, model no. 3500R, for $49.99. Coupon code "ROKUDEAL" cuts it to $39.99. With free shipping, that's tied with our mention from two weeks ago as the lowest total price with seen for a new one. (It's a current price low by $9.) It plugs into your TV's HDMI port and streams 1080p video via WiFi.
Update: Amazon offers it for the same price.