Criteria:
1. 2 or more successful series on same network
2. Longevity (Ed, Sullivan, Walter Cronkite, many of the anchors on 60 Minutes, etc)

CBS
Ed Sullivan
Walter Cronkite
Red Skelton
Jack Benny
Dick van Dyke (DVD Show, New DVD Show, Diagnosis Murder)
Mary Tyler Moore (DVD Show, MTM Show)
Lucille Ball (I Love Lucy, Lucy Show, Here's Lucy)
Bob Newhart (BN Show,, Newhart)
Tim Reid (WKRP, Simon and Simon)
Buddy Ebsen (Beverly Hillbillies, Barnaby Jones)
William Conrad (Cannonn, Jake and the Fatman)
Carol O'Conner (All in the family/Archie Bunker's Place, In the Heat of the Night)
Tom Selleck (Magnum PI, Blue Bloods)
Gerald McRaney (Simon and Simon, Major Dad, Promised Land)
Bill Cosby (Cosby, Kids Say the Darndest Things)
Dixie Carter (Designing Women, Family Law)
Burt Reynolds (Gunsmoke, Evening Shade)

ABC
William Shatner (TJ Hooker, Boston Legal)
John Ritter (Three's Company, 8 Simple Rules..)
Suzanne Sommers (Three's Company, Step By Step)
Tim Allen (Home Improvement, Last Man Standing)
Lee Majors (Big valley, Six Million Dollar Man, The Fall Guy)
Heather Locklear (TJ Hooker, Dynasty, Spin City)
Bob Saget (Full House, America's Funniest Home Videos)
Tom Burgeron (AFV, Dancing With The Stars)
Tony Danza (Who's The Boss?, Taxi)
Teri Hatcher (Lois & Clark, Desperate Housewives)
Katherine Hellmond (sp?) (Who's The Boss?, Soap,, Coach)

NBC
Bob Hope
Johnny Carson
Ed McMahon
Bill Cosby (I Spy, Cosby Show)
Michael Landon (Bonanza, Little House on the Prairie, Highway to Heaven)
John Larroquette (Night Court, JL Show)
Kelsey Grammer (Cheers, Frasier)
David Hasselhoff (Knight Rider, America's Got Talent)
Howie Mandel (Deal or No Deal, St Elsewhere, America's Got Talent)
Joe Rogan (NewsRadio, Fear factor)
Maura Tierney (NewsRadio, ER)
William Daniels (Knight Rider (uncredited), St Elsewhere)

how about nbc announcing icon don pardo who was with nbc as a staff announcer from 1944 until past the year 2000 and today is still in the driect employ of producer lorne michaels still announcing saturday night live at age 95

I would add Carol Burnett to the CBS list.

ABC:

Shelly Fabares (The Donna Reed Show, Coach)
Robert Guillaume (Soap, Benson )
Robert Wagner (It Take a Thief, Hart to Hart)


CBS:

Jim Nabors (The Andy Griffith Show, Gomer Pyle USMC)
Bob Denver (The Many Loves of Dobie Gillis, Gilligan's Island)
Ed Asner (The Mary Tyler Moore Show, Lou Grant)




NBC:

Robert Fuller (Laramie, Wagon Train, Emergency)
Jack Webb (Dragnet, Dragnet 1967-70)
Victor French (Little House On the Prairie, Highway to Heaven)

CBS- Ray Romano and Kevin James

ABC- Tim Allen and Roseanne Barr

NBC- Jerry Seinfeld and Kelsey Grammer

A few more
CBS

Bob Barker
Bud Collyer

NBC
Bill Cullen

Pat Sajak

For ABC: James Brolin (Marcus Welby, M.D. and Arthur Hailey's Hotel)

CBS
The legendary Mike Wallace - well respected investigative reporter for over 40 years (60 Minutes).

ABC
John Ritter - I know he was listed in the op, but I believe Hooperman should be added to John's accomplishments.

CBS
Gene Rayburn

Another for CBS:
Brent Musburger (has done many sports for CBS in his long career, but is chiefly known there as being on The NFL Today on Sundays from 1975-89)

NBC
Hugh Downs
Art Fleming
Bob Costas
Dick Enberg
Jim Perry
Chuck Woolery

ABC
Jim McKay
Keith Jackson
Frank Gifford
Dick Clark