Microsoft's version control system, both available as a seperate product and
as part of the Enterprise edition of Visual Studio. The current version number as of the time of writing is 6.
It supports most common features, such as branching and audit trails, and
provides a SCC (Source Code Control) interface so it can integrate with any Windows
program which uses this interface. It fully integrates with many Microsoft products,
such as Visual Studio, Word, Excel, Access and FrontPage.
As well as accessing it through another product, it can be controlled via
a GUI program known as SourceSafe Explorer (where administration takes place), as well as via the command line. It is not server-based, but relies on a database which is shared between all clients.
It is also available on non-Windows platforms - a Macintosh port is available from Metroworks, and a UNIX port from Mainsoft (essentially just the Windows version compiled using their Win32 porting libraries). If you're not on either of these platforms, or need good remote access, SourceGear provides a web-based version called SourceOffSite.