I apologise for the 'Ad', but I work with schools, so anything that can significantly reduce costs is useful, so I hope this helps others. This is the lowest-cost Dual H-Bridge DC motor controller I've seen.
It's a FAN8100N made by Fairchild and costs 0.81 GBP in packets of 5:
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=6710071
It is a little different to an L293D. It is missing the 'enable' signals.
Otherwise it looks good, with short-circuit, thermal and back EMF protection, and 1.5A output (3A in Parallel), and working voltage 1.8-9V. It has forward, reverse, stop and brake with only two inputs/motor so it will use 2 Arduino PWM pins/motor to drive it (of course, the PWM can be done in software using interrupts).
The only major annoyance is two pairs of pins are 'joined' together to make a 'metal flap' (for a heatsink) which stops them fitting into a breadboard 
It has a funky 'turbo' feature for one motor. When 'on' it connects one motor to a higher voltage, which would then go faster. It is designed for RC model cars 
HTH
GB