This instructable covers the solar heater I made from parts available at the local hardware store (or salvage) for cheap. I have yet to do true empirical measurements on its output/efficiency, but it will raise the temp of my hot tub (~460gal) from 70 to 80 in two sunny days, and keeps it in the 90s during the summer without using the tub's heating element. This allows me to keep it warm and only use the electrical element to boost the temp when I want to jump in (saves quite a bit of $$ on electrical bills), after which this will keep the temp up in the 100s for a day or two on its own. This is the result of a few experimental panels, and the finished product turned out to be about the easiest of them all to do. A more refined version with fewer connections could be made if properly planned out.
It can be built in an afternoon, possibly just a couple hours if you have the parts ready to go. The longest wait time is for paint and sealant to dry/cure.
*Trying to find more of the pics I took while building this, taken over the course of about a year (hence the new look of the wood at the start, and OLD/weathered look at the end)
** Evidently the photo tagging thing likes to move my tags up from where I put them.... working on getting it fixed, for now just imagine them a good bit lower than where they are