In reply to Dave B:
THe only ones that I know that pulse are Exposure and they're not exactly cheap.
Loads of choice for small, cheap lights that flash. The bontrager ones are good - bright, tiny and rechargeable. I have one of those as a permanent fixture on my road bike, but when it's dark and for commuting I use an Exposure one (and a big Lezyne one too) but that still doesn't mean that they see you (viz. £600 bill for wheels driven over in the course of a left hook).