Apple has done a great job at implementing multi-touch technology in their input devices, and the new Magic Mouse and MacBook trackpads are hits for most people who've tried them. Despite this, there are some limitations to the devices which prevent them from being used at their full potential.
While Apple's trackpads support 2, 3, and 4-finger gestures, the application of these gestures is limited to a few functions. Likewise, Apple's new Magic Mouse supports a number of gestures, but Apple has limited which can be used in the software.
There are a couple of third-party utilities and drivers available (some currently in development), which will offer more input options and customizations for Apple's multitouch devices.
This is a small project aimed to enhance the multitouch experience on Snow Leopard machines, both for the trackpad and Magic Mouse. It is very much a beta testing project (and should probably be considered in "alpha" development) so while it is a fun experiment, expect it to be buggy. Despite this, it may work for some people and be fun to check out.
MouseWizard is a very cheap ($2.50) mouse driver that adds a number of options to the Magic Mouse. It gives 'pinch', 'bloat', and 'tap-clicking' behaviors among others to the new mouse, along with alternative function assignments to the various behaviors.
While built for the multi-touch trackpads, this application will work with the new Magic Mouse. TheAppleBlog has some instructions on setting it up for use with the Magic Mouse.
jitouch (thanks to the commenters)
Similar to MouseWizard for the Magic Mouse, jitouch provides a number of enhancements to the multi-touch trackpad (though they claim Magic Mouse support is coming soon). The added gestures include multi-finger sequential tapping combined with individual finger swiping to bring a variety of new functions to the trackpad. The program is only $5.49 for a license.
I've been a long-time fan of USB Overdrive, since it has allowed me to customize practically every mouse I've owned in virtually every conceivable way. The new multi-touch interfaces provide more of a development challenge than single-button inputs, but on the USB Overdrive web page the developer has noted multitouch support will be coming soon. I will keep an eye out for updates on this one.
If you try any of these alternate drivers and input managers, keep in mind that you can always uninstall them if a problem arises by booting into Safe Mode and removing them from your system. Most are simple installations that consist of an extension, a startup item, and a preference pane.