The point of RepRap is to make itself, of course. But sometimes people just want plug-and-play RepRap electronics so they can concentrate on the other aspects of the machine, or just because they are more software or mechanics oriented than electronics oriented.
Melzi is designed to be a complete set of RepRap electronics that should be very cheap to mass produce. It is based on the Arduino Leonardo (Francesco Melzi was Leonardo's pupil).
The Melzi Ardentissimo is a full-featured Melzi board using the ATMEGA644P chip used on Sanguinololu. This is a Sanguinololu firmware compatible board, though does have a couple of extra features the Sanguinololu does not - namely the extra Mosfet for the fan, and a software controllable LED. Melzi has micro SD on board as well.
Melzi with Leonardo
The Melzi [insert cool name here] is a Melzi board based on the new and upcoming Arduino Leonardo. While this board doesn't have the same expansion capabilities as the Melzi Ardentissimo, it may be more cost effective, as the Leo has built-in USB negating the need for the FTDI232RL chip on the Ardentissimo.