Being a Firefox user (I know, I know, and it crashes on me all the time, but still I run back to it ashamedly), I had no idea that there was a gap in the functionality of StumbleUpon (a handy little link-finding and sharing browser plugin) for Safari users -- it only works with IE and Firefox. But Eli K tipped us off that he's trying to bridge that gap with Stumbi, a StumbleUpon plugin for Safari.
It's the very definition of no-frills: it just creates a menu option for StumbleUpon that will let you access the most basic of functions from the Safari browser. And unfortunately, it's not exactly completely easy to get working -- you've got to make sure to install SIMBL (which, helpfully, comes with the binary download), and then the binary also requires a reasonable $2 purchase after 100 stumbles (or, for the more technically inclined, you can download the source and do it all by yourself for free).
But if you just can't live without StumbleUpon, and can't bring yourself to use Firefox to do it, this might be just what you're looking for.