AT&T is currently selling refurbished iPhone 4 handsets through their website. The deals are pretty amazing right now, to be honest. Yeah, there is the whole contract thing and a $36 activation fee, but an iPhone 4 16GB is not a bad deal at $99, refurbished or not.
If you want to save a little more money, and a cheap iPhone 4 is still more than you can or may be willing to pay for a phone, you can also pick up the rebfurb iPhone 3GS 8GB for $29 or the iPhone 3G 8GB for under $20.
There are also iPhone 4 units described as refurbished with “cosmetic blemish,” which basically means there is something on the actual phone itself. Perhaps minor scratches, we’d guess. The good news is that these run even less than the previously mentioned refurbished iPhone 4.