go4computers via eBay offers the Unlocked Apple iPhone 4S 16GB Worldphone for GSM Networks in Black for $619 with free shipping. That's $30 under what Apple charges and the lowest total price we could find, although we listed it for $19 less three weeks ago (since expired). It's powered by the Apple A5 dual-core processor and features an 8-megapixel camera which is capable of recording 1080p video at 30 fps with image stabilization, dual-antenna system for faster downloads and improved call quality on GSM networks, AirPlay Mirroring, Siri voice-activated assistant, 8-hour battery life, and more. It's compatible with GSM networks such as AT&T, T-Mobile, Vodafone, and more; it can't be used with CDMA networks such as Verizon or Sprint.

Note, however, that on some networks such as T-Mobile, the phone will not operate at 3G speeds.