Autotyping and autocorrecting applications can be a huge time-saver, sparing you from having to retype frequently used words and phrases (and even paragraphs and pictures), and they can instantly clean up your most common typos. Plus, if they work system-wide like Typinator does, they can provide you with functionality that might otherwise be limited to your word processor.
Typinator is a best-in-class example of this kind of app, with an easy-to-manage interface, above-average stability, exceptional speed (keeping up with even extremely fast typists using large abbreviation sets, even those whose keystrokes can overlap), and the ability to handle multiple languages (with autocorrection for English, French, and German). You can also search through your "snippets" with a Spotlight-style QuickSearch, which makes large snippet files more feasible. Typinator supports iDisk and Dropbox syncing, and recent updates have improved support for other apps, including Kindle for Mac, Outlook 2011, iTunes 10, and even Gmail in Firefox. Built-in HTML snippets, autocorrection sets, and frequent upgrades make Typinator an even more attractive option.