There are too many brands and sizes to cover them all here, so I picked my favorite ones (which I use and have tested through the years)

store price
Elite 799 air pump

Small pump with good lungs :) perfect for a beginner's needs. Can take up to 3 spawning tanks, maybe more.

$8.00
Elite 802 air pump

Quiet and powerful about twice more air output than the 799. 

$19.00
Elite Optima air pump

The BIG daddy of the Elite line, I love this one  and have 15 spawning tanks hooked up to it. Could hook up more. It is super quiet, has a big knob to control air flow. I highly recommend it. Comes with spare diaphragm :).

$44.00