Earplugs are definitely a simple solution, but have the drawback of not being highly visible to people around you.
When I need to concentrate, I wear my big Sennheiser headphones - often not even plugged into an audio source.
They are very comfortable for long periods, block out most of the noise, and (best of all) they send a clear "do not disturb" message to anyone thinking of interrupting.
Would headphones w/ noise cancellation help? Even if you didn't play music through them?