TotalFinder became a commercial solution not long ago: the full beta development stage was free and gave me plenty of time to test the software. It's an excellent solution, and I have seen almost every single bug disappear progressively.
Even if not a free solution anymore, its source is available on GitHub. I would recommend it on the grounds that it already works wonderfully well, and definitely deserve some support in my (otherwise freeware-oriented) opinion.
If freeware is yet a compulsory feature for you, Geekosaur's link has a few options.
Edit, 2013-01-07: as mentioned in another answer, XtraFinder looks like TotalFinder, but free.