More multitouch gestures
Apple has been bringing multitouch features into Macs for a long time now. The original MacBook Air pioneered the use of the trackpad for multitouch gestures -- pinching, swiping, and the like -- in 2008. Apple has expanded these gestures in more recent MacBook models, as well as in its Magic Mouse and Magic Trackpad peripherals.
In Lion, Apple has promised to bring even more iOS-style multitouch gestures and visual responses to Mac OS X. Among the new gestures demoed on Apple's Lion page are rubber-band-style scrolling, enhanced pinch and zoom functionality, and full-screen swiping. Whether Apple will offer even more advanced gesture support is an open question, but I wouldn't be surprised to see some more in the final release.
If you don't want to wait to get more gestures and capabilities, however, you don't have to: There are several utilities available for getting your multitouch groove on in Leopard and Snow Leopard (but not earlier Mac OS X releases).
First up are two tools that simply expand on Apple's existing multitouch features. MagicPrefs (free) and MouseWizard ($5) add support for multiple-clicking and augment the existing swipe/pinch/drag gestures; they also let you automate a wide variety of tasks, such as copying/pasting, switching spaces, and launching applications using the Magic Mouse. As of this writing, both of these products work only with Apple's Magic Mouse. Support for Apple's Magic Trackpad is planned for MagicPrefs, but no timetable for that addition is available.
Next up are more ambitious multitouch extenders. BetterTouchTool (donationware, currently in alpha) offers the ability to assign custom gestures to perform a wide range of system tasks, including opening and closing windows, invoking Mac OS X features like Dashboard and Exposé, launching applications and websites, adjusting preferences such as sound and brightness, controlling iTunes, and mimicking specific key combinations or mouse functions such as right-clicking. It works with Apple trackpads, the Magic Mouse, and traditional multibutton mice; it can also be used to assign custom keyboard shortcuts.
Jitouch ($7) functions with Apple trackpads and the Magic Mouse, and it includes a library of built-in multitouch gestures for each type of device. Both global and application-specific gestures are available to activate a variety of features and commands, such as switching applications, working with tabs in Safari, activating window controls (minimizing and moving window position, for example), switching spaces, and activating Exposé. Like BetterTouchTool, Jitouch also lets you assign custom gestures.
Another neat option is its support for character gestures; you can assign actions that are invoked by drawing a specific shape on the mouse or trackpad with your finger (similar to the stylus-based Grafitti input on old Palm OS devices). This allows for a lot of customization but is also something that can take a bit of getting used to (and thus isn't for everyone).
Keep in mind that while all of these tools are similar, each one has its own unique variations on what it does and how it functions. Choosing between them is generally a matter of individual taste and needs, including what devices you use. Therefore, you'll want to check out all these tools to find the one that works best for you.