Zak Dolan was an early champion of magic (the first worlds champ, I think) who more or less dropped off the face of the earth......
I can't possibly mention everyone. too many players to think of.
All the members of the old mogg squad made significant developements in magic.
Ryan Fuller, Chris Benafel, and the rest of Alpha Beta Unlimited.com
Massimo Esposito for being the first and only person with a lifetime ban (sleeping with the judge's wife, I believe, was the call) is a magic legend, more than anything.
Kowal also worked with the metagame and as a deck developer and player and things
Frank Kusomoto invented THE magic site, the dojo, which is now sadly closed. It was a golden age for magickers near and far while he reigned.
Chris Senhouse, another player who's recently dissapeared, as a great player/deck developer/theorist. Alot of people fall under this category.
Aaron Forsythe, for being a good writer, excellent player/designer/theorist, and for taking his brother's casual deck and making it a force to be reckoned with....
Andrew Cuneo as a deck designer and player
Andrew Johnson, the same
Anthony Alongi, popularizing multiplayer along with.....
THE FERRET. editor-in-chief of starcity
Nate Heiss, for the average guy thing, in the CMU, and being a nice fella, and a good player/designer/theorist
John Rizzo, for being one of the best writers anywhere, and the biggest scrub in CMU
Theron Martin, for his reports on the metagame, basically defining it, and for his sad, long, exile
Lan D. Ho, Trey Van Cleave, Matt Vienneau, and several other people who are just all around good. Writers, theorists, and players.... too many to list.