Up until this summer, the Canon 580 Ex and EX II were my workhorse flash units, at one point when I was shooting weddings every weekend, I had 4 of them. It was Canon’s Flagship Flash unit, but has been replaced by the 600 EX-RT, which uses longer distance capable Radio Transceivers. There is still a significant stock available (while supplies last) and probably will be for the next 1-2 years.
If you look around, you should still be able to find 580s on Ebay.
If you want to learn how to use it, look no further: Canon 580 Speedlite Crash Course DVD