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.
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 L-M
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; etc.
Firefly on The Batman, Speedy and Paco on Batman: The Brave and the Bold, Duke on G.I. Joe: Renegades, 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. (But I might move the entry someday.)
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.
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.
Astro on Astro Boy, Lonette in Cool World, Dexter in seasons 3-4 of Dexter's Lab, Madame Foster et al. on Foster's Home For Imaginary Friends, Ophelia on The Life and Times of Juniper Lee, Zadavia et al. on Loonatics Unleashed, The Flea et al. on Mucha Lucha, Mrs. Wakeman on My Life as a Teenage Robot, Irma on W.I.T.C.H., etc.
Mirren, Helen See actresses M
Deep Thought in The Hitchhiker's Guide to the Galaxy; Dean Hardscrabble in Monsters University; Nyra in The Owls of Ga'Hoole; Tuya in The Prince of Egypt.
Montalban, Ricardo See actors L-M
Head of Council in The Ant Bully, Guitierrez on Freakazoid!, Señor Senior Sr. on Kim Possible.
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.
Murray, Joel See actors L-M
Carl Bitterman on Baby Blues, and I guess the animated series based on the movie Beethoven (though I don't remember that well), Don Carlton in Monsters University.
Garfield in all kinds of specials plus the series Garfield & Friends, Peter Venkman on The Real Ghostbusters, and Larry the Crash Test Dummy in a series of PSAs.
Newbern, George See actors N-R
Frank Tennyson on Ben 10: Alien Force, Sephiroth in Final Fantasy VII: Advent Children, Superman on Justice League.
Norris, Daran See actors N-R
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.