Working in open plan offices and other noisy environments has made it obvious to me that I (probably this is true for everybody else, too) can't do my best work without a certain amount of privacy. In fact, “isolation” would not be too a strong a word to describe the most productive environment for me. When my various workplaces fall short of the monastic austerity and complete solitude I imagine to be ideal, which is always, noise cancelling headphones have been an enormous help. I'm using a pair of Audio-Technica ATH-ANC1 at the moment and am very pleased with them, although their on-ear design may be physically uncomfortable for some. Of course, noise cancellation is far from perfect and voices still come through, but filling the headphones with some kind of audio signal helps make up the difference. Music is often too distracting on its own, but I have a few favorite audio loops that are just enough to mask off the world without wearing on my concentration themselves:

• SimplyNoise is a website with white, pink, and brown noise generators, any of which can be amplitude modulated by a slowly varying sinusoid. I suspect the oscillation prevents my mind from getting too good at filtering the noise out and picking up voices again.
• RainyMood.com offers a recording of a rainstorm with occasional thunder. This audio signal is certainly more structured than the other noise sources I use, but the sound of rain is so natural that it's not usually able to break my concentration.
• My absolute favorite, though, has to be this 24-hour long (!) YouTube video of the ambient engine noise heard on the Enterprise from Star Trek: The Next Generation. I grew up with ST:TNG, and I have to admit there's something very comforting about getting some work done with this particular soundtrack!

For more on the relationship between sensory stimulation and workplace productivity, I recommend Susan Cain's Quiet: The Power of Introverts in a World That Can't Stop Talking, which, among other influences, has lately given teeth to my desire for a quiet and focused workplace.