head-fi.org is THE headphones website. Some of the best under $100 are the Grado SR-60/80 and the Koss Porta-Pros. If you like earbuds, check out the Sennheisser mx 400/500, although the stock iphone ones arent terrible at all. For in-ears, I like the sony ex-71's for comfort although there are probably better ones out there. If you can step up your budget a bit, look for a used pair of sleek sa6. Those are my current favorite, with a built in physical eq system, etymotics comparable sound, detachable/replaceable cord (allows you too use a thin cord without being afraid of it ripping) and the option to get the W1 and go wireless. This wireless option is much better than bluetooth as it transmits over wifi, giving you enough bandwidth to not have to compress the audio. And it's probably the most comfortable set of phone's i've used (comparable to the sony's). Sennheiser HD280's are also great, and you can get them at guitar center for about $100. Don't be afraid to buy used headphones, they are one of the best things to buy used. This was all written assuming you like rock or pop music; for classical check head-fi.com