http://www.heelys.com/

They're a new kind of skate.  Each shoe has a single wheel concealed in the heel.  You can walk on them normally, or shift the weight to your heels and skate on them.  There's a bit of a learning curve to it; it requires completely different muscles and muscle memory than either walking or skating.  But they're a very convenient conveyance, and a crowd-pleaser, too.  You can even pop the wheels out and walk without the wheel protruding.

''I bought a pair, and loved it.  Unfortunately, since there are only two wheels in contact with the ground, it doesn't do well over particularly thick, deep cracks in the sidewalk, as exist in my suburb.  It may still be useful for the work commute, since there is no sidewalk half the way there.''  --NickBensema