Think about it.
A piece of wire, bent in a simple shape, that holds paper together.
Even with computers, email and photocopiers, every office still needs paper clips.
Wish my great-great grandfather had invented it, and patented it.
A better invention, and almost as simple: the pencil.
While we’re doing office products, I’m going to go for sticky tape.
Ha! None of that would be worth a shit without PAPER! 
I’m vaguely remembering a book I read in childhood, The Invention of Paper. Were there Muppets in it? Hmmmmm
Personally, I think slinky springs are pretty cool. Especially since you can use one as a paper clip in an emergency… although I’m not sure what that emergency would entail, exactly.
Also, buttons are good.
~ Isaac
Actually, the pencil is very much NOT a simple invention. You may want to check out The Pencil: A History of Design and Circumstance by Henry Petroski, an engineering professor who has written a number of popular books on design and the engineering process. It’s a 450 page book on the interesting technology and personalities that have been involved in the development of the modern lead pencil.
The corkscrew?
It’s even simpler!
Ladies and gentlemen, I give you…The Inclined Plane!
The site pales in comparison to this one:
http://lectureonline.cl.msu.edu/~mmp/kap4/cd095a.htm
Oh, come on, give me the old Archimedes Screw every time.
Well… you know what I mean. 
Screw Archimedes!
One more vote for the corkscrew.
Or maybe Fire…
Sharp, pointy objects, in all their various incarnations are both simple and useful.
No votes here for the wheel? Or even for the Log Roller?
Dani
a guy in my town patented the little plastic clip thing you see on bread wrappers and and other bags. there is a whole multi million dollar company that does nothing but make those little plastic things.
Kwik-Lok i think their called.
String.
With string, you can tie things up, sew things together, produce a net, make a million things, and then invent cloth. Yay for string!
Useful stuff, string, no trouble there. Just the right length for a million household uses! Attaching notes to pigeons’ legs, destroying household pests… destroy 99% of known household pests with pre-sliced rustproof ready-cut easy to handle Absorb-I-Tek Stringettes!
Tooth brush and sticky notes.
wheel, no question.
And if that turns your crank (and related to the OP), Petroski also wrote a book called The Evolution of Everyday Things which goes into great detail on the invention, design, and evolution of the paperclip…
I can’t figure out why, but that’s really fun.