Adorama offers the refurbished Nikon WU-1a Wireless Mobile Adapter, model no. 27081, for $24.95 with free shipping. That's $14 under the lowest total price we could find for a new one. It allows you to send photos from your camera automatically to your smartphone, or to use your smartphone to control your camera. It features 802.11n wireless and compatibility with Android, iOS, and multiple Nikon cameras.
A 90-day warranty applies, but it's unclear who backs it.