Previous to Safari 6, you could hit the Delete key to act as a back button. For some reason, when Safari 6 was released with Mountain Lion, Apple decided to remove this function. It may not have been used by many people, but I for one used to all the time. When I found it missing in Safari 6, I was hoping someone would find a work around, or a way to re-enable it.
As it turns out, there is a terminal command that will restore the functionality of the Delete key in Safari 6.
1. Open Terminal (/Applications/Utilities)
2. Copy and paste the following in Terminal:
defaults write com.apple.Safari com.apple.Safari.ContentPageGroupIdentifier.WebKit2BackspaceKeyNavigationEnabled -bool YES
3. Restart Safari (For best results, it is a good idea to quit Safari before you do this, but it is not necessary.)
Enjoy using your Delete key as a backwards button again. If you ever wanted to undo this, just repeat the above command, but replace YES with NO.