One obvious one is a LJ client, another would be a dict(ionary) client.
But then I came up with another idea that would be useful. A site where you could feed it the name of a book (or CD, DVD, or perhaps articles), and the software would tell you where you could find a copy, whether your own book collection, your local libraries (or perhaps your friends libraries), or a bookstore.
Also it'd keep track of the things you searched for, so if you were in one of those places you could see what you might want to pick up.
A coworker after listening to that idea, suggested something similar for products, where you could feed in a UPC code, and it would tell you what the going price is. A useful extention for the price search would be to collect the price for where you currently are.