Stumbi is an effort to make StumbleUpon feel more mac-like. It currently comes in two flavours, both of which are completely free:

The client

The Stumbi application is a dedicated application for browsing StumbleUpon, and is our recommendation.

Requires Mac OS 10.6 (Snow Leopard)

The plugin

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
These apps are provided free of charge, without support. We can provide them free because 2% of stumbles are sponsored.