As a student at Princeton, Szaky kept his startup alive by convincing judges at business-plan competitions that he could make plant food from "worm poop" (or waste). Today, his natural plant food, which comes in recycled soda bottles, can be found on the shelves at Wal-Mart and Home Depot -- with sales expected to jump to $5 million next year.

Age: 24
Location: Trenton, N.J.
Revenue: $461,000
Employees: 23