Sennheiser HD 202
It's tough to find a decent pair of full-size, closed headphones for under $50--they're generally pretty painful, in terms of both their fit and the sound they produce. The HD 202 are clearly an exception to that rule. Not only do they sound good, but with good padding on the headband and earpieces, they're also fairly comfortable (if a bit bulky for portable use).
Thee HD 202 won't beat out more expensive, higher quality headphones, but if you don't mind a bit of emphasis in the bass, for $25 you get a solid pair of closed headphones.--Dan Frakes