Interestingly, the sixth most popular OS (after Vista x64) is Mac OS X 10.6.3. Adding the different versions together, OS X's usage share totals about 5%. Not too shabby, at least if you consider the Mac version of Steam has barely been around for three months.
Other noteworthy stats: AMD's Radeon HD 4800 series are now the most popular graphics cards among Steam users, but AMD's graphics usage share is only 33% (Nvidia's is 59.11%). 1680x1050 is the most popular resolution, almost a third of users have quad-core CPUs, and Intel still dominates with a sizable 72% CPU usage share.
Source: Tech Report