Mac: Although we have a favourite text expansion utility already, xType has the advantage of being free while still maintaining a comparable feature set to the paid competition. It's not yet ready to claim the top spot in the text expansion category, but it's a seriously good alternative if you're looking to save some money.
xType works like pretty much any text expansion app. You enter in the shortcut you want to use, the text you want it to expand to, and then type that shortcut to test it out. On top of the basic functionality, xType can also save formatting information, add images, and handle simple variables like the date and time. When it supports clipboard functionality and, perhaps, input dialogues, it'll be serious competition to the big guys. For now, it's a really great free alternative.