I use good’ole TextExpander, and it’s my favorite – of the few I’ve tried. The others weren’t even good enough to remember the names of.
I recently noticed under the Language & Text setting in SnowLeopard 10.6.1 there is a replace “blank” with “blank” feature under the text tab. I had yet to get it working because I believe it requires a restart, and I didn’t feel like restarting at the time. However, it might be a free alternative to third-party text-expander programs out there (if you can get it working that is).