This is a simple Safari extension which turns the +1 logo in Google+ into a 1UP mushroom. It also plays the appropriate sound effect, but you can turn that off in the options. (Go to the Extensions tab in the Safari preferences.)
This Safari extension is based on Jason Mobarak's Chrome extension which does the same thing in Chrome.
There's a git project at http://www.cs.hmc.edu/~tcb/1UP/1UP.git if anyone wants to hack on it.