I want to improve my development environment and now I'm thinking about using a versioning system in the future. The problem is that I haven't much experience with versioning systems (a bit with Perforce). What Versioning Systems for Website Development are available for free?
Any of the modern version control systems can be used for web development. The choice of which system to use is basically just personal preference, OS choice / integration and the tools you like to use.
I'm using Subversion, with VisualSVN on the server side and TortoiseSVN on the client. I like how it's integrated with Windows Explorer. Others that are gaining a lot of traction lately are Git and Mercurial. I think these three are the most widely used today but there are a lot more.
Wikipedia, predictably, has a large comparison of revision control software, but the main thing it comes down to is whether you want to always use a centralised repository (either on your development computer or a server), or to be able to commit changes while not in contact with your main repository.
The benefit of the second option, using distributed revision control is that if you normally check your work into a server, but are out on a plane or otherwise without an Internet connection, you can still commit changes to your local copy of the repository, and the software can merge with the other repository (or several others) when you next get a chance.
The main centralised systems are CVS (older) and Subversion (abbreviated SVN), while the big players in open-source decentralised revision control are Git, Mercurial (also abbreviated as the chemical symbol for mercury, Hg), and, more recently, Bazaar (similarly, BZR).
For many of these, there is a Windows GUI integration named "Tortoise<X>" where <X> is the software name.