OK, I went though IMDB and pulled the following voices and who acted them. There are many other bit voices not included. From the list, you can see that several of the actors have provided a large number of characters. I don't know if actors get paid by the voice or just a flat fee per episode, but some are irreplacable. The ones, like Katey Sagal, have distinctive recurring voices.
Anyway, her is the list, no where complete:
Billy West was responsible for Phillip J Fry, Zoidberg, Professor Farnsworth, Zap Brannagan, Richard Nixon's Head, Leo Wong
John DiMaggio: Bender, Elzar, Mom's son Ignar, Mr. Panucci, Yancy Fry Sr, Robot Santa, Elves, Henry Kissinger's Head
Tress MacNeille: Mom, the Grand Midwife, Mrs. Fry, Slurm Queen,Tinny Tim
Maurice LaMarche: Kif Kroker, The Donbot, Clamps, Calculon, Morbo, Lrr, Horrible Gelatinous Blob, Mom's son Walt, and Hedonism Bot, Hyper-Chicken, Robot Devil
Phill LaMar: Hermes Conrad
Katey Sagal: Leela
Lauren Tom: Amy Wong, Inez Wong
Dan Castellaneta: Robot Devil (also voiced by Maurice LaMarche)
David Herman: Scruffy,Reverend Preacherbot,Crow T. Robot ,Slurms McKenzie, Niedermeyer,Dr. Ogden Wernstrom
http://en.wikipedia.org/wiki/Futurama