Surprised that no-one has mentioned Perforce. It's very reasonably priced, works really well and the diff and merge tools are excellent.
We chose it for our team of four developers because it is fully and properly supported and wasn't going to break the bank. Clearcase, TFS and Rational were way too much money for a small team. You'd need quite a few developers to cost justify them. With the experience of Perforce over several years I'd happily recommend that over the others anyway.
And.... I haven't looked recently but you used to be able to run a limited copy for free. I used it at home for some personal projects.
$0.02, Paul