There are arduino clones with "real" (AFAIK) Atmel chips for the main CPU, but "different" chips for the USB/Serial converter. (A real Arduino Uno uses a second programmable Atmel chip - an ATmega16u2. The original Arduinos used an FTDI chip, and I've seen clones with Prolific (probably counterfeit Prolific) or this currently popular CH340 chip. This is OK for most arduino applications (the idea of the 16u2 was that it would be reprogrammable to allow the board to be USB Mouse/Keyboard/Midi/etc; things other than a serial port. But that doesn't seem to be as popular as people thought it might be.)
You do have to be careful WRT drivers; there have been reports than some chips don't have MacOS drivers, and Prolific doesn't support some older chips in Win8, and has apparently cracked down on counterfeits.
I sorta like the CH430 chip, mechanically. USB/Serial converter in 16pin SOIC. Hobbyist friendly, if you could buy them anywhere. (?)