Riccardo Ettore's has a long history; it's been around since 1987, which is longer than I've been using a Mac. In case you've forgotten, here's how it works. You supply TypeIt4Me with pairs of abbreviations and expansions (such as "ty" and "TypeIt4Me"). Then TypeIt4Me watches you type and substitutes expansions for abbreviations in just about any application.
As I explained, you'd think that TypeIt4Me would be impossible to reimplement under Mac OS X, given the latter's deliberate resistance to system-level hackery. But not so. Earlier Mac OS X  of TypeIt4Me were ingeniously implemented as an input manager, a mechanism that evades this resistance. However, whether because this implementation was proving somewhat limited and unreliable or because  have recently been tainted with ignominy thanks to their proven potential as a security hole, TypeIt4Me 3.0, which was released late last month, has been rewritten to use the  instead (like its rivals,  and ).
So now, when you install TypeIt4Me, you see - or rather, you don't see - a background-only application, controlled by a System Preferences pane and optionally manifesting itself as a menu extra (an icon and menu at the right side of the menubar). When you type an abbreviation followed by whichever of three dozen delimiter characters you've specified (e.g. "ty@", where "@" is the delimiter), TypeIt4Me observes this fact through the Accessibility API and tells the application you're using to select those characters and paste in their place the expanded version, restoring the clipboard afterwards. A special expansion syntax enables you to perform many useful additional tricks, such as specifying where the insertion point should be after the paste, or inserting the original contents of the clipboard at some point in the pasted material (good for entering HTML opening and closing tags, for example).
Abbreviation/expansion pairs are stored in files that can live anywhere you like, and an abbreviation file (or an individual abbreviation) can be associated with a specific application. TypeIt4Me can also be disabled for particular applications, and in a pinch you can even temporarily turn off TypeIt4Me's automatic behavior entirely, by choosing Pause from its menu. In that case, you can still enter an expansion by choosing its abbreviation from the menu.
In my view, TypeIt4Me 3.0 represents a significant architectural revision; it is elegant and simple, appears reliable, and truly makes for a fine blend of flexibility and user confidence. TypeIt4Me 3.0 requires Mac OS X 10.3 Panther or higher, and is a Universal Binary. It is a, and costs $27 ($9 to upgrade form any previous version); you can try it free for 30 days.