Text expansion is a wonderful thing — especially on a mobile device. Typing a shortcut that expands into a larger amount of text can save a ton of time and effort. While TextExpander exists for iOS, it only works in applications that support it and is of limited use. If you've jailbroken, however, you can use Xpandr which works system-wide and costs significantly less.
Xpandr is remarkably simple. You open the app, tap Edit, and then tap plus to add a new snippet. A snippet consists of two things: a shortcut and what that shortcut expands to. You enter both, save it, and then you can type that shortcut in virtually any text field and it will expand. Text expansion is certainly not a new concept, but to have it work system-wide on an iDevice is pretty great.
Xpander will cost you $US2 in the Cydia Store (that's $US3 less than Text Expander). To find it, just search Cydia for Xpandr and go through the usual motions to purchase and install it.