MacFarlane, Seth See actors M
Stan Smith and Roger on American Dad!; Tim the Bear on The Cleveland Show; Peter, Stewie, Brian, Quagmire, Tom Tucker, et al. on Family Guy. He's also done some live-action acting, as well as writing/producing/directing.
MacNeille, Tress
Dot, et al. on Animaniacs; Avatar Yangchen and Hama in episodes of Avatar: The Last Airbender; Antonia Chillingsworth in Dark Fury; Fang on Dave the Barbarian; Mom, Linda the news anchor, et al. on Futurama; Osono in Kiki's Delivery Service; Obaba in Nausicaa of the Valley of the Wind; Oroku in Pom Poko; Sarah Ravencroft in Scooby-Doo and the Witch's Ghost; Agnes Skinner, et al. on The Simpsons; Babs on Tiny Toon Adventures; Queen Victoria (elderly) in Van Helsing: The London Assignment; etc.
Mako (aka Makoto Iwamatsu)
Iroh on Avatar: The Last Airbender; main title narrator on Dexter's Laboratory; Aku on Samurai Jack; Achoo in an episode of Duck Dodgers that parodied Samurai Jack; Splinter in TMNT. I also saw him in a live-action role in an episode of Greatest American Hero.
Marin, Cheech See actors M
Ramone in Cars; Mad Hog in Hoodwinked Too!; Banzai in The Lion King; Tito in Oliver & Company; Lencho on Santo Bugito; himself in an episode of The Simpsons; Hare in Tales from Earthsea; etc.
Marsden, Jason
Firefly on The Batman; Speedy and Paco on Batman: The Brave and the Bold; Duke on G.I. Joe: Renegades; Max in A Goofy Movie; Binx in Hocus Pocus; Fingers on Kaijudo; Danger Duck on Loonatics Unleashed; Haku in Spirited Away; Richie on Static Shock; Billy Numerous in a couple eps of Teen Titans; Tino on The Weekenders; Matt on W.I.T.C.H.; Chase Young on Xiaolin Showdown; etc. I also know him from some live-action stuff, such as Almost Home, but mainly I know him as a voice actor, which is why I'm putting his links here.
McConnohie, Michael
Schwartzwald on The Big O; Chojiro Sasakibe, Ryuken Ishida, Kurodo, et al. on Bleach; the Narrator on Bobobo-bo Bo-bobo; Charles zi Britannia on Code Geass; Captain Jurgens on Eureka Seven; Babbo on MÄR; Ramba Ral on Mobile Suit Gundam; Dr. Boyer, Franz Bonaparta on Monster etc.
McFarland, Mike
George Kaminski et al. on Case Closed; Master Roshi, et al. on Dragon Ball; Jean Havoc on Fullmetal Alchemist; Buggy the Clown et al. on One Piece (FUNi dub); Shota Jinnouichi in Summer Wars; Cain Nightroad on Trinity Blood; etc.
Miller, Lara Jill
Min and Lulu on Blood+; a Tachikoma on Ghost in the Shell: Stand Alone Complex; the title character on The Life and Times of Juniper Lee; Haul on Rave Master; Scheris Adjani on s-CRY-ed; Koko on Zatch Bell!; etc.
Montalban, Ricardo See actors M
Head of Council in The Ant Bully; Guitierrez on Freakazoid!; Señor Senior Sr. on Kim Possible.
Morris, Phil See actors M
Bud in All I Want for Christmas Is You; Joshua Sweet in Atlantis: The Lost Empire; Damon Gray on Danny Phantom; Vandal Savage on Justice League and in Jusice League: Doom; King Faraday in Justice League: The New Frontier; Imperiex on Legion of Super Heroes; Doc Saturday et al. on The Secret Saturdays; etc.
Morrow, Kirby
Goku on Dragon Ball Z, Trowa Barton on Gundam Wing, Billy Katagiri on Gundam 00, Miroku on InuYasha, the title character in Mosaic, Prince Aika on Oban Star-Racers, Cyclops/Scott Summers on X-Men: Evolution, etc.
Newbern, George See actors N-O
Frank Tennyson on Ben 10: Alien Force; Sephiroth in Final Fantasy VII: Advent Children; Superman on Justice League.
Norris, Daran See actors N-O
Count Spankulot on Codename: Kids Next Door; Cowboy Andy et al. on Cowboy Bebop; Vincent in Cowboy Bebop: The Movie; Cosmo, Mr. Turner, et al. on The Fairly Oddparents; J. Jonah Jameson on The Spectacular Spider-Man; various voices on Star Wars: Clone Wars; Spottswoode in Team America: World Police; etc.