A research lab at one of the big chip makers issued me earmuffs, as they did to all employees. Note: This is a research lab, which looks a lot like any cubicle environment at a company like Google, Microsoft, etc. This worked very well, and to this day, I consider noise-blocking earmuffs to be part of my office supplies.
Good noise-blocking earmuffs are better than earplugs. If they are of good quality, they will be more comfortable than all but the best headphones. Be careful, because many of these earmuffs are designed to block loud noises like jet engines, while letting in conversations. You do not want something that lets conversations in, but instead, muffs that block everything.
The best set I have found (other than very expensive examples) are the Bilsom Viking V3 earmuffs. See http://earplugstore.stores.yahoo.net/bilsom-viking-v3-1.html
as one example.
When I am wearing my earmuffs, I can barely hear my phone ring. If someone walks up behind me and talks to me, I do not know they are there.
P.s. One more important consideration is one way to block noise is to block air movement. Some inexpensive earmuffs do this, but it causes pressure issues in your ears, similar to pushing your hands against your ears (painful!).
You can tell whether a set of earmuffs is good by putting them on and then pressing the muffs tighter into your head. If the pressure goes up like you are in an airplane, these are cheap. The Vikings will NOT do this.