I have seen a couple write-ups similar to this one. Basically you get $50 worth of material (tie wraps, irritation hose, + couple 2x4s), and make a solar heater from it. This one is a little different where the author is using a pond pump to move the water around. I wonder how effective that pump is for under $50….of course later on the author mentions he swaps the cheap pump out for an $80 one. My 2 cents, perhaps putting a 2nd pool pump in that could be had for $50 and under on craigslist might be a better approach :). Either way this sounds like a good build. I like it since my pool pump is in an area that does not get allot of sun. So putting something in-line with it would not be a good idea.
http://www.instructables.com/id/Pool-Solar-Heater-or-Optical-Illusion-for-Entertai/