Nikon has the longest flange to focal-plane distance, so you can't use lenses meant for other 35mm lens-mounts on Nikon cameras (the lens would be too far from the sensor/film to focus to infinity). The exception might be if you were doing closeup/macro photography and didn't care about infinity focus. There are some medium-format lenses that can be adapted for use on Nikon cameras, though.

Canon has one of the shorter flange to focal-plane distances, so lenses for other mounts can more easily be adapted to Canon EOS (even with the extra spacing of the adapter in place, the lens is still close enough to the sensor/film to focus on infinity).

Adapting Nikon lenses to use on Canon cameras is a pretty common thing to do. For instance, lots of Canon shooters use adapters with Nikon's wide-angle lenses, which are particularly well-regarded.

One note about adapting Nikon lenses for use on Canon bodies. You'll either want to get the older lenses that have a physical aperture ring; or you'll need to get a special "G" adapter that allows you to set the shooting aperture on Nikon's 'G' lenses that have no physical aperture ring. Also, there's no adapter I know of that will work with Nikon's new PC-E lenses, since they have a completely different aperture control mechanism (currently only the three tilt/shift lenses are PC-E, though).

Another consideration is that adapted lenses are going to be completely manual-focus, and you may also lose in-camera metering (best you can hope for is probably spot- or center-weighted metering in manual exposure mode, maybe aperture mode if you're really lucky). So unless you have a specific reason for wanting to use a specific lens, it's probably not worth the trouble. I wouldn't recommend getting an adapter just so you can share lenses with friends. If all your friends shoot Canon and you want to be able to share lenses, you should go with a Canon DSLR.