I code almost all day wearing a pair of these. Without music playing, they mostly block out a conversation on the other side of the room. With music, the outside world is nonexistent. The biggest downside is a weak plastic headband casing. There's a strong metal band inside, but the plastic casing will eventually crack (it happens to most owners -- might be dependent on head size). That said, I wrapped mine with clear packing tape a couple years ago and they've been fine since. They're comfortable enough to wear for an hour or two on, 15 minutes off, repeat.
Another great sounding set is the Audio-Technica ATH-M50. I bought my brother a pair for $100 new on eBay. They're more durable than the HD280s, and fold up better for portability.
Avoid the HD202 unless you have small ears. Also avoid anything from Grado; they're totally open (lots of sound leaks in and out), and are mostly appropriate for rock (they have a very "colorful" sound -- not at all faithful to the source material). Don't go anywhere near Bose. You'll get a lot better sound for a hell of a lot cheaper from just about anything made by Sennheiser, Audio-Technica, Beyerdynamic or AKG.
This is a great source for info: http://www.head-fi.org/forum/ Try not to get too sucked into the hi-fi tomfoolery though, for $100 you can get a great set of cans, and a laptop or mp3 player will be a sufficient driver. A lot of the higher-end gear will require an amp to sound decent, and there's some wickedly diminishing returns once you pass the $100-200 point. There are a lot of conflicting reviews on that site, but there's seems to be a general consensus about certain models (the ATH-M50 is widely respected, for example).
[For the best noise blocking (aside from in-ear), you want a set of closed "circumaural" (around the ear) headphones. Open-back cans don't block any external sound, so you have to crank up the volume to drown it out.]