The Sennheiser MX400/450/500/550 are generally considered to be pretty good. I have a set of 550's myself, and they're pretty fun to listen to.
If you want something larger (and better-sounding), look at the Koss KSC-75. They're clip-ons that are supposed to sound fantastic for the price, and you should probably be able to find them for around $20 at Radio Shack or Circuit City. Koss' PortaPro and SportaPro headphones also use the same drivers, and should sound similar.
And if you can stretch your budget a bit, you might want to check out the Sennheiser PX100. I haven't heard them either, but they're supposed to be excellent. I'm not sure how much they go for in the US, though.