Tape 
 
There are numerous types of tape out there: 
  
Masking Tape — 
 Used for temporary projects, like painting.  Gets horrible and gooey if left too long, especially in the sun.
   
Electrical Tape — 
 Besides electrical use, it's not too bad as a sealant tape (like an emergency repair on a leaky trap under the sink) because it's rubbery and stretches.
   
Duct Tape — 
 Fabled in myth and legend, this stuff is highly overrated.  The only thing it's really good for is what it was invented to do in the first place; seal heating ducts.  There are wildly different grades, from some real junk to the ultra-sticky black stuff found at marine stores.
   
Mirror Tape — 
 Double-sided tape, originally used to hold mirror panels to walls, now used everywhere.  The nice thing about mirror tape is it often peels right off a few months down the road if you want to change something.
   
Teflon Tape — 
 Thin, white rubbery tape used to seal pipe threads.  A marvelous invention.
   
Heat Duct Tape — 
 This has a chrome look to one side.  It's a heat-resistant tape used to seal things like the exhaust vent for a gas dryer or furnace.
   
Plumber's Tape — 
 This is that flat metal 'tape' with all the holes in it that's strapped around your water heater to keep it from falling over in the event of an earthquake.
 
And that's not counting cellophane tape, adhesive and medical tape, packing box sealing tape, and all the rest.
On the subject of tape, although it doesn't have any use in regards to repair, a nod should be given to Dymo Label Tape, and what a valuable role it plays when it comes to keeping things organized.