I wanted a low cost digital watch and a bit of research turned up fascinating information about an old favourite, the Casio F-91W which first appeared in 1991.
It's a simple, readable, watch with a battery life of more than five years.
The Wikipedia article reveals that this watch is reported to be favoured by those interested in building time bombs! Surely this is a testament to its reliability and hack-ability.
Picked one up for AU$21 on eBay and can recommend it, for telling the time at least.