I use a pair of Bose active noise cancelling headphones (Quiet Comfort 15, over ear). I work in an open plan office with many people using the phone or talking across the room. I'm fairly distracted by this and these help, but it doesn't exactly silence outside noise. In fact, for some people their voices seem more pronounced when the noise cancelling is on. However they're pretty good quality, comfortable to wear and can truly deaden the sound of air conditioning and keyboards.
Major downside is that you have to turn on noise cancelling to listen to music - which is powered by a single AAA battery.
Major downside is that you have to turn on noise cancelling to listen to music - which is powered by a single AAA battery.