SourceGear Vault (Version 1.2.2 as of this writing) is a version control system for Windows developers. If you've used Visual SourceSafe, Vault is a similar (but much better) product. It is built entirely on the .NET Framework with a SQL Server 2000 backend storage system (as compared to VSS's file system storage system).
A key feature of Vault is its backward compatibility to VSS. Another is its inherently better reliability, due to the fact that it uses a transactional database for its storage, rather than a file system (which in VSS tends to get corrupted and lose things). If you've used VSS, or another source control system like CVS or many others, Vault should be pretty familiar. Vault is made by the same company who sells SourceOffSite, a product that allows VSS to work well over the Internet, so these guys have a solid understanding of VSS, including its users and its shortcomings.
Another nice feature of Vault, which I haven't personally taken advantage of as yet, is that it supports access to the repository via Web Services, so you can build solutions around it, extending it as needed to fit into your development process.
You can find out more about Vault on its website: