1024*768 is a lot less common than it once was but it is still common enough to be considered for things such as web design (i.e. "it should look right on 1024*768 or higher, and degrade gracefully on lower"). It is the most common resolution on non wide-screen laptops for instance. Most new non-widescreen monitors are 17" or 19" and have a native resolution of 1280*1024 (5:4 aspect ratio), but larger models often have larger resolutions (those these aren't overly common). Widescreen resolutions are a lot more varied though. In my limited experience 1680*1050 (16:10) is most common in new monitors at the moment, closely followed by 1920*1080 (16:9) for screens over 22". For small screens (and some larger screens that are primarily intented to be TVs) you will find that 1280*720 is not uncommon. The many mobile and otherwise small-form-factor devices available muddy the waters much further though.
The lowest in general use where I work is 1024*768 (a couple of laptops, the newest being ~18 months old). The lowest on the desktop is 1280*1024. On my desk at home I have 1920*1050 and 1050*1680 (a widescreen monitor rotated 90 degrees is a wonderfuly useful thing sometimes). The smallest though is my netbook at 1024*600 (or sometimes 600*1024), which is used at times for both work and play.
If you are asking because you are planning some sort of design that might be screen resolution/size sensitive, I would recommend that you aim quite small as your lowest common denominator and try make sure your design looks good (or at least good enough) on a range of screen sizes. Long gone are the days when there was only two or three screen resolutions to worry about in the general populous - there are many screen sizes that are common enough to be considered popular.