Surveying the landscape of the Inc. 500 is a little like reading a science-fiction novel. It's populated with both the familiar and with strange new concepts you've never considered. Eventually some of these companies will become household names, such as Inc. 500 alumni Microsoft and Oracle, but thus far most of the private companies that have grown the fastest since 2001 have been flying under the radar of the mainstream business press. Which is one reason you can expect to find some companies on the list that have business models that didn't come from any textbook.

Ever thought of giving stuff away in exchange for signing people up for trials of other stuff? That's how Gratis Internet (No. 18) took in $20.5 million last year. Got a great product but a not-so-great marketing plan or distribution system? Dynetech (No. 127) will supply both--in exchange for half your profits. Intelliseek (No. 476) gets paid to measure buzz online, no doubt capturing some of the work of New Media Strategies (No. 154), which gets paid to create it. Thinking big but thinking you can think bigger? Maybe you need a session in the Rockies with Starizon (No. 226), where you can run your notions by Galileo, Jefferson, and Einstein. You get the idea. For more surprising and inspiring finds, read on.