Here are some I can think of off the top of my head...
If you like Jet, you'd also like the Arctic Monkeys and the Fratellis.
If you like Massive Attack, you'll like Blackfilm, Ewun, Future Prophecies, Soman, and Technical Itch.
As for the Crystal Method, you'll probably also like The Freestylers, Junkie XL, Overseer, Pendulum, The Prodigy, Propellerheads (guys who did the matrix theme song),
If you like Tonic, you'll also like Oasis and Cracker.
If you like KT Tunstall you'll probably like Lily Allen and maybe The Submarines.
You might as well get some Queens of the Stone Age. Also, Thievery Corporation is a must have for any music collection.