That's offered for an unlimited number of strings by MS Word. That isn't free, of course, but if your question is about word processing and you already have that program, you wouldn't need anything else. I didn't check other word processors such as LibreOffice.
It's different if you want it to work in, say, Notepad and the command processor also. But without the context of your question I don't know if that's the case.
I'm looking for a program that lets you define short key combinations for long words or numbers; e.g., when you press zzk the word kleptomaniac is automatically typed if you've defined that combination. I'm now using Keyboard Booster. It has some flaws but isn't bad. However, the free version only allows 15 combinations. Does anyone know of a free downloadable one? I've searched but can't find anything. I'm running Windows XP sp3 on an HP desktop microtower.
BTW, the search link in the welcome post doesn't seem to work.