8. Continuous build tools
The more collaboratively minded developers among us like the way continuous build tools like Jenkins, Hudson, and other "continuous integration" servers help us work together for the betterment of the whole. These tools put all code through a continuous stream of tests, alerting developers about problems with code they checked in some 10 seconds ago, keeping everybody moving toward the same goal.
Tools like Hudson or Jenkins aren't new -- there have been a number of slick proprietary continuous integration tools for some time, but the emergence of open source solutions encourages the kind of experimentation and innovation that comes when programmers are given the chance to make their tools better.