Even the 4GB 1st generation iPhones are selling for ~$200 on Ebay, so getting a used one out-of-contract is well above $100.
I've never heard good things about the iPhone clones.
There's the "HiPhone"
http://www.tomsguide.com/us/ip...hiphone,video-160.html
The newer i32 version is supposedly a bit better, but still the reviews are centered on how buggy it is and how the voice and sound quality is too quiet.
There's the CECT P168 and M88. They are both running Windows Mobile 6 - which is a bit ironic. They generally have not completely terrible reviews, but you can read for yourself:
http://www.amazon.com/CECT-QUA...-WINDOWS/dp/B001UBIM5S
There's the Meizu M8 and Miezu MiniOne. These are perhaps the best iPhone clones available... but their price generally reflects this, and these are well above the $100 limit.
Personally, I would recommend avoiding these clones and instead buying a 3G iPhone at the new $100 price point, or if you want avoid AT&T contracts, then increasing the budget a bit and picking up a 4GB iPhone on Ebay.