I'm looking into this right now also. In order to figure out what timer you need you'll need to say what you want it to do and what size pump you have. You can find the pump hp rating on the sticker located on the side of the pump.
For example my filter pump is a two speed pump 2hp/.33hp, and I have a 3/4hp buster pump for the cleaner. I need to control both speeds on the filter pump and also the buster pump. I'm looking at the Intermatic P1353ME timer. It allows control for all three pump scenarios.
Intermatic is the brand most people go with. They have *many* different models. Which one you want depends on how you pump is wired. If your pump plugs in with a standard 120v plug you could probably use something like the HB31R. If the pump is 240v and is wired in directly you would need something like the WH40. There are many different ones, different voltages, different Amps, battery backup, multiple circuits, etc.
I have the Intermatic HB31R and it is really not made to last more than a year or two in my opionion I am looking at the Intermatic T101P and will just hard wire the pump to it. Oh by the by, thanks for the feedback.
i've had the t101 model for the past 2 years. it runs 24/7 year round including in the cold winter and works like a charm. i considered less durable intermatic models to save a few bucks when i bought it, but an "industrial strength" model with a strong lifespan seemed the way to go rather than cheaping out (heck, still the cheapest piece of equipment in my pool install by a long shot is how i looked at it)