I like the Sony MDR-ZX300. I own a pair (I'm listening to them now, in fact!), and I think they sound very good for sub-$30 headphones! In fact, I'd say they handle most music--from Rush to Lil Jon--with aplomb! While they're clearly not audiophile-quality, I think they stack up well against more expensive headphones, including the $400 Sennheiser HD 650 that a friend let me listen to: I'd say the Sony headphones are about 95% of the Sennheiser, for less than 10% of the price!
My favorite value-priced headphones are listed at my blog (search The Froogal Stoodent).
The focus here is on value, rather than pure sound quality.
Not everyone on this forum agrees with me on these headphones, but I really like them--just a matter of taste, I suppose!
(And no, I'm not paid by Sony--I just really like these cans!)