Sennheiser PX100. I was against them a while ago in favor of returning to the Koss Portapros (which I had before I got them), but going back showed me how much better the PX100s are. They're a perfect portable phone, great sound and you can get them for under $50. If you've just been using cheaper headphones like the standard over ear ones or Panasonic Shockwave, the difference in audio quality will completely amaze you. You'll never go back after these.
I've heard great things about the HD-497s as well. I love closed ear headphones (while these technically aren't, they cushion around your ear keeping a lot of sound in which is basically what I'm talking about) and I really wish I could find these where I live. They're supposed to have slightly better sound quality than the PX100s, as well as obviously not leaking as much sound.