I've tried a variety of active-noise-reduction and/or noise-blocking solutions:
ANR
Sennheiser PXC-250: very nice overall. No, they do not kill all the noise from an airplane or a data center. But they do help enough to reduce travel fatigue a good deal. I love them as "motel room" phones: they are great at cutting out air conditioner hum, traffic noise from outside, etc. Sound-wise, they are pretty nice; I like them a lot better than either the PX200 or PX100. Drawbacks: battery unit in the middle of the cable is awkward and tangle-prone, especially combined with the floppiness of the foldup design and the double-sided cord.
Sennheiser HDC-451: semi-open design. NC similar to PXC-250. Non-folding design, cord enters on only one side, stiffer cord. Pretty good sound, more of an open-headphone sound than the PXC-250. Less convenient to stuff into a carryon when travelling, but IMO preferable to the PXC-250 for non-travelling use.
Bose QuietComfort II: (I don't own these, but tried them in an Apple store with my own iPod and amp.) Outstanding noise reduction, top of the class. Sound is nice enough on some tracks and grimace-inducing on others. They were not for me, but will readily satisfy people who have been listening on earbuds or $10 headphones up until now.
Sony MDR-NC5: Augh!! Get it off me!! Not much noise reduction either.
Noise-blocking
Ety ER-4: I love them. You've already read all the reasons why elsewhere in these forums.
Sony MDR-EX71SL: Every once in a while I come out of the closet and announce on head-fi for all to hear: I actually like EX71's. No, they are not Ety's. Yes, the bass is overblown. No, they don't block all that much sound, just a few decibels...but it's enough to make a nice difference. They're cheap, they're comfortable as heck, they block a bit of noise, and I even think they sound nice. Oh, and did you mention gaming? I get really outstanding positioning cues from my EX71's, better than any of the above (though I notice it much more in DVD movies than in games).
Sennheiser HD280: They block more noise than the EX71, less than the ER4. They clamp my head way too hard, no way I'd wear them through the course of a flight over 1 hour. For me the sound is OK on some material but grimace-inducing on anything complex. Gonna sell mine.
AKG K271: Blocks some noise, but far less than HD280. Maybe about as much as EX71. Doesn't fold up, would be pretty awkward in a carry-on. Sound is sweeeeeet, but needs an amp pretty badly IMO. A better amp than my Supermini or portable PIMETA. Raif's DAC1 drove it nicely...
Beyerdynamic DT231: blocks a bit of sound coming in (say, a bit less than EX71). Does not block much coming out. What#&%!, I thought these things were supposed to be sealed! Well, they're not. So if you're looking for headphones that won't bug your workmates, these aren't they. Too bad, though, because the sound is kind of usefully profiled for a work/noncritical-listening perspective...they have lots of bass and an extended top end, so even turned way down, you hear enough thump and enough detail to get a nice impression of the music.
Honorable mention:
Etymotic ER-20 hi-fi earplugs. Carry them on the plane, and enjoy quieter takeoffs and landings, when your "portable electronic devices" are verboten. Take them to concerts and action movies, and get the full experience of the music and explosions with less volume and less headache. Keep them in your pocket when you know a fire drill is coming, and saunter down the stairs while everyone else in the stairwell is grimacing and holding their hands over their ears. Cheap and highly recommended
Curious about:
Sony MDR-NC11: They appear to be essentially EX71's plus active noise reduction. Hmm! Seems like I'd like them, but haven't tried them yet.