First, as with iOS, Lion's dictionary will begin to learn new words after you've clicked the X in the autocorrect pop-up three times. Of course, you can also highlight any word and use a contextual menu to make the dictionary learn a word as well.
Most of the multi-finger gestures available in Lion (and Snow Leopard) vary whether you're using a trackpad (be it built into a MacBook or Apple's Magic Trackpad) or Apple's Magic Mouse. In almost every case, the same features and gestures are available, but use one less finger on the Magic Mouse -- for example, swiping back and forward between pages in Safari uses two fingers on a trackpad but just one on the Magic Mouse, and swiping between Spaces uses three fingers on a trackpad but just two on the Magic Mouse.
Speaking of gestures, you can enable/disable many of the gestures using the Trackpad or Mouse panes in System Preferences. You can also select from a handful of alternate actions to be assigned to each gesture.
Second, you can easily view the definition for any word by double-tapping on it with three fingers if you're using a trackpad. The same feature works with Apple's Magic Mouse but uses two fingers.
Safari's new Downloads menu
A well-known Safari enhancement in Lion is a redesigned Downloads list, which displays as a pop-up menu in the Safari toolbar rather than as a separate window.
That's obvious, but a subtle addition to this new Downloads list is the ability to drag items from the pop-up menu directly to the desktop or a folder of your choosing, moving them out of the Downloads folder. This can be done after items are downloaded or while they are still actively downloading.
New screen-sharing tricks
OS X has offered screen sharing since Leopard. Up till now, you could log into a remote Mac and control its screen as if you were sitting in front of it. The typical vehicle for starting a screen-sharing session was iChat, where you could request permission to control a friend or family member's computer.
Screen shares could also be started via the Finder's sidebar as well as the Screen Sharing application inside the CoreServices folder. (CoreServices contains a number of system-level files that are best left untouched, as well as Screen Sharing. It's located inside the Library folder in the System folder at the root level of your startup drive.)
Previously, when beginning a screen share through the Finder or Screen Sharing, you would share control of the Mac with the active user (if someone was logged in and using the Mac at the time) or log in to the remote Mac by entering the name and password of a local user account on that Mac.
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