It’s becoming increasingly rare that developers are working without some sort of version control system in place. There are a lot of different solutions out there, ranging from the very expensive ones like Team Foundation Server (which does a lot more than source control) to a variety of open source supply chain management (SCM) applications.
At my day job, I use Team Foundation Server, and I just started setting up Subversion in my home office for personal use. What are you guys using? (For the sake of convenience, related applications are grouped together in the poll answers.)