The Mercurial source control system for (nearly) every one of my free software projects. Recently, I began using it to manage all of my web sites. If you are like me, you likely have a web server and SQL server of some sort running on your desktop, permitting you to work locally.
I make many frequent changes to my sites. Sometimes things don't go so well for various reasons. I may need to roll back changes that I made to something, get my site mirrored in a hurry if its getting an abnormal amount of visits, change hosts in a hurry, lots of other things. Mercurial helps with everything.
Now I can work locally and just 'hg push' my changes to my server, make changes on the server and easily 'hg pull' them to my local copy, or merge if I've modified the same files in both places. I can quickly grab a whole copy of my web site to setup a mirror on many other systems, or even quickly produce patches to send upstream to free software projects (like Wordpress, phpbb, etc).