Headphone Recommendations

These are my recommendations at various price points for anyone shopping for headphones. I'm an audiophile and a headphone enthusiast, and I've listed to a lot of cans.

Which headphone is best for you will depend on a number of factors. You should answer these questions for yourself before selecting a headphone:

Is noise isolation important for you? Headphone fall into one of two catagories: open or closed. Open headphones don't isolate at all—you can hear what's going on around you, and those around you can hear your music—but open headphones tend to have higher sound quality than closed headphones of similar price. Closed headphones provide some sound isolation, but not all closed headphones will completely block in-coming or out-going noise. Studio headphones and in-ear monitors (IEM's) tend to provide the most isolation.
Will most of your listening be on-the-go, or will it be mostly at my desk or easy chair? Larger headphones tend to sound better, but are inconvenient for travel and may not be properly powered by portable players. There are some very good and expensive IEM's, but if you listen mostly at home full size headphone will provide a better value. You can't beat the size, convenience, efficiency, and isolation of IEM's if you listen while commuting, though.
Do you listen to one music genre much more than others? Some headphones are good for many genres, others excel at only one genre. Grado headphones are great for rock, but not very good for classical, for example. Etymotics have great clarity, but perhaps not enough bass for hip-hop fans. Because of my eclectic tastes, most of the headphones I list below do at least reasonably well in a variety of genres. If you listen primarily to one genre, there may be better headphone for you.

Of course, it's best to listen before you buy if possible. Also remember that there is some correlation between price and sound quality, but it's not always true; there are definitely bargains and rip-offs in audio. Bose products, for example, are scorned by audiophiles not because they sound terrible in absolute therms but because they cost 3-4 times what products of similar audio quality cost from other manufacturers.

Under $20: Koss KSC75
Around $80: Sony MDR-V6 (If you want portable in-ears, the Apple dual-driver monitors are OK. The Etymotic MC5 might be better, but it's a little bass shy.)
$120: Audio-Technica ATH-M50. Note that the prices for these fluctuate a lot. If you're patient, you should be able to find them for $120 or a little less.

Most of the headphones below need an amp (dedicated headphone amp, or quality receiver) to sound their best. They're probably not worth the money if you'll be using them out of an iPod or sound card.

$80 plus your time and patience to mod them: Fostex T50RP. Unmodified, these are not worth consideration, but when properly modified they compare favorably to $400+ headphones. See the head-fi thread.
$200: Grado SR225. Recommended only if you mostly listen to rock, but strongly recommended in that case.
Under $350: Denon AH-D2000. It looks like Denon has been more tightly controlling the advertised price of their headphones recently, but you should be able to find them for less—perhaps much less— if you call around. These sound good stock, but can be improved by modding. The after-market lambskin ear pads from JMoney are a nice upgrade.
Around $400: Sennheiser HD650. The price of these has spiked recently. I'd be mildly annoyed to pay more than $400, but jump on them if you see them closer to $300 or $350. The slightly cheaper HD600 is just as good, though less "fun" sounding.

Over $1000, either the Audeze LCD-2 or the Sennheiser HD800 are highly recommended. I don't own either, but have heard both. The LCD-2 is less of a "wow" headphone than the HD800, but it's a warmer, bassier, and may be more enjoyable for long listening sessions. The HD800 is certainly amazing though.