Valve periodically asks users to respond to a survey that automatically collects information about their computer hardware, to better understand their target audience for new games. This represents all Steam users who said "yes" to allowing Steam to collect information about their computer.
http://www.steampowered.com/status/survey.html
Highlights:
64.5% of respondents have broadband (768kbps or higher) (less than I expected)
44.5% have 512 to 999 MB of RAM, 29.57% have 1 GB or more
46.5% AMD, 53.5% Intel (LOTS more AMD than I expected)
75.4% 1 CPU, 24.1% 2 CPUs, 0.5% 4 CPUs
57.7% nVidia, 35.2% ATi, 7.1% Other
Top 5 graphics cards by popularity:
GeForce 6600 (w00t for me)
GeForce 7600
GeForce FX 5200
GeForce 8800
Radeon 9600
DirectX 9 Shader Model 3 rendering path is dominated by nVidia cards, Shader Model 2 rendering path is dominated by ATi cards.
2.31% of respondents have DirectX 10 GPUs.
ATi respondents have somewhat more recent drivers than nVidia respondents.
83.6% have a 4:3 display, 17.4% have widescreen
51.75% of nVidia respondents have PCIe 16x
1.19% of respondents have more than one GPU (thank God)
Of those, 99.4% have 2 GPUs, the rest have more
95.56% of multi-GPU users are using SLI, 4.44% are using ATi Crossfire (I had no idea Crossfire was doing so poorly)
The vast majority (I'm not going to add it up) of respondents use onboard sound.
90.3% XP, 8.0% Vista (more Vista than I expected)
60.4% of respondents have their language set to English.
95.4% of respondents have DVD-ROM drives (so can we PLEASE never release another game on multiple CDs?)
The only thing I wish they had shown that they didn't show is whether users were running a 32-bit or 64-bit operating system.
