Version Control was not a big thing as long as I was doing freelance development. But once I got into the process of working with a team I realised that a team programming without source control is useless.
I started using CVS with the WinCVS client. Then moved on to use Subversion with the TortoiseSVN client.
I spent most of the last 2 years in Microsoft world, so Team Foundation Server was the system.
Right now I’m on Ruby and we are using Perforce which is actually damn good!
The user interface is damn simple although it might look a bit funny.
It’s one of the systems used by development teams which work on various platforms. I can check in code from my Windows box and get latest on my Linux box / server. This is really cool !!