The answer is different per platform:
Mac: Almost all cameras are supported. Try the free CameraTether App; if that one works, so does PictureMe. In general, all cameras that continue to function when connected to a mac with a USB cable and have PTP (Picture Transfer Protocol) Mode support, will work.
PC: Both Canon and Nikon Cameras are supported, other brands are not, as each manufacturer has their own driver and proprietary method of tethering. If we get enough requests to add a certain manufacturer, we can add support to the feature request list.