I made a solar water heater that heats the pool during the day. It pumps the water up to the roof, runs it through some black plastic tubing and back down into the pool. An automatic timer turns it on and off every day, so it only pumps the water when the sun is out.
It's also worth mounting the panels in such a way so that they won't blow off of your roof. Take the time to properly secure your panels!
For more information you can read my blog: http://www.danielbusby.com/sustainability/a-quick-look-a...r-water-heater/
Here's a link to the Lily Pad Pool Warmer video: http://www.youtube.com/watch?v=HGdxvDM5IKg
Also, here's how a friend made his own pool heater with black tube from Home Depot: http://whereisholden.blogspot.com/2012/04/thermal-solar-pool-heater-summary.html