Roll Your Own Service
You say you don't like the way that Craigslist, Flickr, or Google Maps functions? Just change how each presents its data, or combine the information on one site with that on another. ProgrammableWeb lists over 200 mashups, including a Google Map of Hindu temples, a Flickr screen saver, and a matchmaking tool for the HotOrNot.com dating service that turns you into a virtual yenta.
You can modify one of the thousands of existing applications on the site and have a slick program of your own devising running in minutes. Ning even hosts the site, posting ads alongside your application in exchange. If you're feeling geeky, the service lets you alter the application's underlying PHP code.