I second the Beyerdynamic DT770 Pros. I've been using them for the past 3 years in the workplace and they work wonders. If you really want to go overboard with them, and care about excellent sound quality, I suggest getting a DAC and tube eventually.

I have the Sennheiser HD380 Pros for this purpose (originally worked in an open office with hard ceilings and floors, now work in a home office with kids). They work quite well. I haven't compared them to active noise cancelling, but I'm pretty satisfied. If I need total isolation, I put in foam earplugs, then put on the headphones, then play white noise over them.

One thing I would recommend is getting an aftermarket set of earpads, as they're far more comfortable and they do a better job of sound isolation. I just got some pads by Brainwavz because the originals wore out, but I wish I'd done it a while ago because of how much better the isolation is.