Off the top of my head: 1366×768 on laptops, 1280×720 and 1920×1080 (although the former doesn't see much use on computers)... other than that not sure which resolutions are considered "common".
Steam's hardware survey is your best guess (click "Primary Display Resolution" in the list at the bottom to expand the full list).
You really shouldn't assume certain resolutions will be available though (nor specific ratios, for that matter, it could be just about anything). Also most people will frown upon if you ever go fullscreen and change the resolution, or if you can't alt+tab out of it without the program minimizing in the case of multimonitor systems.
PS: wtf 640×480 usage just went up.