Stumbi is an effort to make StumbleUpon feel more mac-like. It currently comes in two flavours, both of which are completely free:
The Stumbi application is a dedicated application for browsing StumbleUpon, and is our recommendation. Requires Mac OS 10.6 (Snow Leopard) | The Safari plugin is mostly functional but no longer being developed due to changes brought in by Snow Leopard. Requires Mac OS 10.4 or 10.5 and Safari 4 |