TextExpander is a popular and reasonably priced autotype/autocorrect program--a productivity-enhancing piece of software designed to save you time and keystrokes by automatically typing out frequently used words and phrases (such as signatures and greetings) and instantly correcting your typos. TextExpander's nearly invisible interface is simple: you type a short, user-specified abbreviation, and the app expands the abbreviation into a longer "snippet," whether a word, a phrase, or even an image or an entire e-mail. (You can even prompt TextExpander to execute a chunk of AppleScript.) You can customize your snippets and manage them in groups, and TextExpander comes with several groups built in, including ones for HTML and CSS, as well as a TidBITs AutoCorrect Dictionary with more than 2,400 common misspellings. You can also embed snippets within snippets, for example to insert the correct time or date.
TextExpander works across all your applications, or you can set the program (or just particular snippet groups) to work only in certain applications. You can use a menu bar drop-down to view and enter snippets, and extensive preferences let you easily search and sort through your snippets and tweak TextExpander to match your work style. It has options for different expansion delimiters and hot keys, to create new snippets and to turn TextExpander on and off. You can also assign a hot key to create new snippets from within applications, so you don't have to leave the application at all, and another hot key will let you quickly edit the most recently expanded snippet.
Overall, TextExpander has solid functionality and lots of nice extras, including the ability to sync snippets between multiple Macs with MobileMe, and collaboration-friendly features that let you share and automatically update snippet groups via a single URL. It's definitely one of the best options available for this type of app.