Here you can find download and install instructions/files for running android on your iPhone. Is it free? Yes Is it usable? Not really...yet. There is still a lot of work to be done to get it to a place where it can be used as your main iDevice OS.
You can download iDroid v 0.2 at Releases - iDroidWiki and get full installation instructions Installing iDroid - iDroidWiki. If there are any newer sources, please share. I think someone has gotten froyo working on a 3G but didn't have time to research it.
It's quite a straight forward process and doesn't affect your iPhone OS installation so after you try it out simply delete the android files and your iPhone is back to its original state (you basically dual boot android and iPhone OS.)
While it's not in a state where you can use it as your main device, it still has enough functionality for you to try it out and get a feel for the OS. I had it running and had phone, multi-touch, text, and web available for me to play around with. The biggest issue is power management, or lack thereof, which causes your battery to go from full to dead in an hour or two. There are also work arounds for other stuff like virtualizing an SD card which you can try out if you are so inclined.