There are many ways to do that. The cheapest way would involve a PC-laptop, a $10 USB/parallel port (most of the parallel port can be used as TTL I/O port), a home brewed software and some extra components depending on the flashguns you want to use.
Originally Posted by cliveh
If you don't want to use a PC-laptop, the next option would involve the use of a multipurpose commercial micro-controller board (e.g. adruino).
Third option, make a dedicated board (micro-controller based).
You might also do it in an analog way with monostable (NE555 circuits) but I don't recommend this solution
If you have no experience with software programming and electronic it won't be an easy project , however if you are happy with the first solution I might help you with writing a piece of code to control your flashguns through the parallel port.