Mercurial
Mercurial is a free open source command line source control program. There is a GUI frontend called TortoiseHg.
Personal
I used Mercurial at my work for several years before we switched over to Git. I found the program to obnoxiously over-complicated, but I find Git to be even worse.
Command Line Options
Show current status of local files compared to local repository.
hg status
Adds un-monitored files to local repository.
hg add
Removes files from your local repository which you have deleted.
hg remove --after
Commits local changes into local repository.
hg commit -m "Description."
Pulls remote changes to local repository.
hg pull
Merges pulled changes into local repository. Conflicts will be listed.
hg merge
If conflicts occur, resolve them manually, then run this.
hg resolve --mark
Finalizes the merger.
hg commit -m "Merged."
Pushes committed local changes to remote repository.
hg push