Rotifers
No, I don't consider microorganism "pets" in the conventional, or even the Unconventional sense. Still they are fun to observe as long as you have a microscope.
My microscope is ancient and my slide is the glass of an old frame.... Still here are a couple of interesting videos of rotifers (Rotaria).
http://www.youtube.com/watch?v=amuAeCOZNjE
A rotifer making its way off stage...
http://www.youtube.com/watch?v=GXn1cTCzOP8
Try to observe the particles around the rotifer's head moving as it creates a stream