CowBoom offers the used Microsoft Xbox 360 250GB Slim Console for $119.99. Coupon code "XBOX20250" cuts it to $99.99. With free shipping, that's $10 under our November mention of a used unit and the lowest total price we've seen for any 250GB Xbox console. (For further comparison, it's currently $150 under the best we could find for a new one.) It features a 250GB hard drive, 802.11n wireless, and HDMI.
Note: No warranty information is provided and this used unit may have dents or scratches and be missing parts or accessories.