It’s a lot easier to write software in a noise-free environment, or predictable consistent background noise. A good set of headphones is a must, and an iPod application or two can be really handy.
White Noise by TMSoft is currently my favorite. Their boat rocking, railroad tracks bumps and various types of rain are great for relaxing just the right amount.
I also highly recommend the ‘Pirates of the Caribbean’ movie sound track, and just about anything composed by John Williams.
Update: My friend Rob pointed me to the incredibly awesome Trance Around the World Podcast. Highly recommended.