How about the marketplace? How about an application to help you run your marketstall: put items up for sale, find other people's items for sale, organise escrow, print out shipping labels, track the parcel as it goes. Broker a deal between buyer and seller and make a cut! Napster for classified advertising.
(This isn't quite the marketplace of Google and the Semantic Web. Close, but I'm talking more front-end, and more Q1 2003 -- if they wanted.)
Sidetrack. Here's the problem with online authentication. Real Life authentication cascades from trusted sources. Birth certificate begets passport begets driving license begets credit cards begets: tube pass, letters sent to your house. And everything based on authentication upstream, ultimately to the great philosopher who makes the mother/child dichotomy (um, cuts the umbilical cord) and signs a slip of paper. You can't fake that. Then further back, that's based on the authority of the doctor; your passport is issued by an authoritative source; and so on and so forth back through history. All of these systems interdependent.
But online authentication isn't like that. There's no history. So we need to bootstrap off RL authentication, when real life things are involved. And, since it's required, what better than iMarketStall, or MSN My MarketStall, or whatever they call it? If a thousand people say that your username accepts payments made to a Visa card in such-and-such a name, that's a pretty good start.