I use a pair of earphones[0] covered by industrial ear muffs[1]. For comfort reasons, it's important that the earphones are not pressured by the surrounding ear muffs. I haven't found any inner ear headphones that meet this constraint. Sometimes I'll swap out the earphones for ear plugs for double ear protection / white noise.
I'm not a fan of open office plans, but this combo lets me tune out everything. Coworkers have gotten used to pinging me or waving to get my attention.
Edit: Drowning out distractions by turning up the volume leads to hearing damage. A better alternative is to eliminate ever hearing external noises. However to prevent incoming noise, ear muffs create a tight seal around your ears. My wife is uncomfortable with the pressure for extended periods of time. I wear them hours daily, and especially love them for sleeping on planes.
[0]: JBuds J2 affiliate link (the white color is $7):
http://www.amazon.com/gp/product/B001GS8FZC/ref=as_li_ss_tl?...
JBuds J2 non-affiliate link:
http://www.amazon.com/Premium-Noise-Isolating-Earbuds-Headph...
[1]: 3M Pelter H10A Optime 105 affiliate link:
http://www.amazon.com/gp/product/B00009LI4K/ref=as_li_ss_tl?...
3M Pelter H10A Optime 105 non-affiliate link:
http://www.amazon.com/3M-Peltor-H10A-Optime-Earmuff/dp/B0000...