unamped, open, no amp:
Koss Ksc35. You can't wrong with these imo. About 30$ directly from Koss.com + shipping (12$ worldwiede shipping). Decent sound and nice bass (taking their size into consideration). Very comfortable, too.
Other options: senn px100, porta pro (both are around 50$)
less portable but still open, no amp needed (although an amp can always improve the sound) and <100$: grado sr60, grado sr80, sennheiser 497
unamped, closed, no amp: senn px200 ( i personally dont like these but some people do) and maybe the sony v6 (not v600) but i'm not sure if these can run unamped.
I just started off just like you, thinking of getting a pair of portable headphones for around 100$ and ended up buying a pair of Sony Cd3000 today.....
i personlly would recomend the Koss Ksc35. I have tried the porta pros (bass was too boomy and overshadowed everything else), px100 (2nd best, but ksc35 was nicer
), px200 (definately didnt like these, not enough bass, weird highs), sennheiser 497 (excellent highs and good resolution for the price, bass was a bit lacking, not as portable as the others, not as comfortable as the other options). haven't listened to the grados yet, but they are supposed to be a bang for the buck, though not very portable.
some nice reviews:
http://www5.head-fi.org/forums/showt...ighlight=ksc35
http://www5.head-fi.org/forums/showt...ighlight=ksc35
or just search the forum