I listened to the Sennheiser 201's just yesterday, and they were quite impressive for $25. They are a closed design, so they wouldn't bother those around you. The cord was very, very long IIRC. Surprising amount of bass, and a lot of refinement for that price.
The other choice would be the Sennheiser PX-100, sold online for about $40, although I've seen them as high as $80 at Circuit City. They sound like they could sell for $80, so half that is quite a bargain. If everybody here voted, these would probably take top honors in the sub-$50 class.
Don't let their small size fool you, they perform beyond their diminutive size. Very lightweight, can hardly feel them on your ears, should be able to comfortably wear them for hours. However, they are an open design, and those around you will be able to hear what you are listening to. Very impressive phone considering its size at any price, but when they're $40, the PX-100 are, IMHO, one of the very best deals in all of HIFI-land.
Both have a two-year warranty. The Koss Porta-Pro have a lifetime warranty, but I've not heard those. The Senn 201's feel like they are much sturdier, with far more bulk, than the PX-100's - but both have the same warranty.
Good Luck!