Today I made a software purchase, ChatterBlocker
I downloaded the demo and used it yesterday and it worked very well. The price used to be $40, but they recently halved it to $20. It is a must buy at that price for anybody that works regularly in an office environment.
I run it on my laptop and listen using a pair of Phillips SBC HN110
noise-cancelling headphones. The headphones are nice because they cover a dynamic range that includes human voices around 1 KHz.
The two together create a pretty impenetrable sound barrier. Assailing my ears without this combo are an air conditioner, electric fan, several computer fans, office chatter or about ten officemates, and some traffic at the loading dock.