Basically, Multiclutch allows you to assign custom keyboard shortcuts in a given app to a given gesture. Want swipes to change tabs in Safari? Done. The same in iChat? Done. Want zoom-in to open emails in Mail, zoom-out to close windows in every app, and a swipe down to bring up Quicksilver? Done done done.
MultiClutch works by installing a simple input manager that catches gesture events, looks to see what shortcut you’ve defined for it in the frontmost app (if you haven’t defined anything, it behaves in the standard manner), and performs that shortcut. You use a System Preference Pane to customize gestures with an interface similar to the shortcut-customization table in the Keyboard & Mouse pane. You can ‘bind’ gestures in a given Cocoa app (due to the nature of input managers, Carbon apps are not supported) or globally.
In addition to the four swipe directions, zooming in and out and rotate in either direction, I’ve been experimenting with ‘combo’ gestures. Right now, I’ve added the ‘Zoom in, Zoom out’ gesture (ie in one fluid motion) and visa-versa, with more perhaps to come if I find them to be intuitive and useful enough.

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