We've been using SVN for years and are extremely happy with it. It runs on a VERY underpowered Linux server and does exceptionally well (meaning you don't need a fancy server for it). I have scripts that back it up every day to secondary disk (in case one disk bites the dust).
In order to choose a source control solution though, you need to ask yourself a few questions:
0. Do you prefer a centralized or decentralized repository? Each has its benefits and drawbacks.
1. Can you afford to pay for scalability (if your company grows)?
2. Do you think you'll require quite a bit of support (open source has forums, paid for software has direct support)?
3. Need to support multiple platforms?
Once you answer those... you should be able to pick one out from the lists of available solutions (I'd choose from the widely used ones... Git, Mercurial, SVN, TFS, IBM stuff).
http://en.wikipedia.org/wiki/List_of_revision_control_software[
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