I've been a fan of TextPander for a while, but way back in the day I used to use TypeIt4Me for my auto-expanding text snippets needs. The developer, Riccardo Ettore, has been writing Mac software since before OS X and he's got a new version of TypeIt4Me out that adds a couple of particularly interesting new features.
First TypeIt4Me can now automatically correct spelling typos using the built-in OS X dictionary (you can also use other typo databases). In addition, Autocue allows you to define templates containing variables or "cues." So for instance you can define a template that includes variables for first and last names in separate fields. When you expand the template it will automatically highlight the variable text and allow you to start typing the names. This could come in extremely handy if you're in the habit of typing boilerplate text that nonetheless needs to be tailored to specific purposes (answering support emails, for example).
TypeIt4Me is $27 ($14 academic); upgrades (for purchases before Aug 1, 2006) are $9 and a demo is available.