Some places to research:
comp.software.config-mgmt
[link|http://www.cmcrossroads.com/|http://www.cmcrossroads.com/]

Based on my research (including comp.software.config-mgmt):

BitKeeper is expensive, and pretty a no-go for small companies. They're not interested unless you're got at least 15 seats or so.

Perforce is $750/client, everything included. You can run a two named user system with all the add-ons for free, so it's easy to evaluate. Perforce seems to have a good reputation on the newsgroup.

AccuRev has a very interesting concept, Streams, that does break new ground. It starts at around $750/client, and the Java GUI is kind a clunky, but it could be worth it, e.g. I suspect doing a lot of branching would be easier in it than most. (I did an eval of it; for a commercial system I'd seriously consider it).

CVS and variants are the open source standard. One guy was plugging CVSNT on the newsgroup (claiming it was better than Subversion).

There are some potentially interesting ones under development, such as Monotone [link|http://www.venge.net/monotone/|http://www.venge.net/monotone/] Darc [link|http://abridgegame.org/darcs/|http://abridgegame.org/darcs/] and Codeville [link|http://codeville.org/|http://codeville.org/]

I'm still evaluating version control for personal use. Since cost is an issue, and I'd like good documentation, stability, and a reasonable user base, I'll most likely go with Subversion. (Subversion has a book available, is past V1.1, and seems to be picking up some stream).

Also remember that the overall Software Configuration Management / Version Control process is very important.

Tony