Subversion is good and free. Perforce is good and not free. You don't have to convert to .mif, you can check in .fm and .book files as binaries. You can't merge binaries but who's going to merge a .mif file anyway?
The one other you might want to look at is Git for Windows, which is a distributed system, reportedly has some advantages for peer workgroups. I haven't used it. Losers: CVS is free and not great. SharePoint is not a CVS. Good luck getting it to do anything except host MS Office-format docs. Documentum is an enterprise solution for huge organizations. MS Visual SourceSafe was bad but it's EOL, haven't used the successor, which might be worth checking out if somehow you have licenses already.
