Have you ever accidentally deleted your single player world? I know I have, and I now know a way to recover them. (For Windows 7 and Vista Only!)
Step 1: Press Start, go to run, and type in “%appdata%” (Without the quotation marks.)
Step 2: Open your “.minecraft” folder.
Step 3: Go to your “saves” folder.
Step 4: Right-click and go to “Properties”
Step 5: Go to “Previous Versions” and click on the time/date you would like to go back to, and then click “Restore”
Hope i have helped you recover your saves, but please note that this will not always work on all pcs. Keep that in mind.