The first thing I thought of when I saw this thread was Dave Foley on Newsradio and Kids in the Hall. Though I don’t know if that counts since he played many characters on Kids in the Hall.
So nevermind.
It was Citizen Kane! Citizen Kane! It was Citizen Kane!
Judd Hirsch “Taxi” and “Dear John”
Richard Mulligan “Soap” and “Empty Nest”
Would Billy Crystal being on “Soap” and later regularing on SNL count - as he mainly played “characters” as opposed too appearing as a skit player?
Valerie Harper “Rhoda” and “Valerie” (wasn’t there some wierd lawsuit between her and the network that kept her name for the show after she left the show).
Tom Poston in “Newhart”, “Grace Under Fire”, and various other good character parts.
Brent Spiner was a continuing guest on Night Court, as some sort of hillbilly, and of course Data on Star Trek:TNG"
damn… Mrs. Columbo … didn’t watch it, just remembered she was Mrs. Somebody who had a show.
Wasn’t Richard Boone, Pallidan on Have Gun Will Travel also Hec Ramsey?
Dirk Benedict was in Battlestar Galactica and The A Team.
Bea Arthur for Maude and The Golden Girls.
Betty White on The Mary Tyler Moore Show and Golden Girls
confession, I thought James Arness was Hec Ramsey, when I Googled to verify, then I saw it was Richard Boone, and I thought of Pallidan.
Peter Scolari on “Bosom Buddies” and “Newhart”
Goodness, if yer gonna mention “Benson” (twice!), might as well add Ethan Phillips (later as Neelix on “Voyager”).
George Peppard had Banacek and The A Team
Bill Bixby was in The Incredible Hulk, The Magician, My Favorite Martian, and I think a few others.
Saltyre said
He was also in It Takes A Thief way back in the '60s.
…and Young & the Restless.
Jimmy Smits, part of a larger ensemble: LA Law, NYPD Blue.
How about Jeff Conaway from Taxi and Babylon 5?
The jury is still out on Scott Bakula, for Quantum Leap and Enterprise.
I have another- John Ritter, for Three’s Company and Hearts Afire (?)
Markie Post: The Fall Guy, Night Court, and Hearts Afire.
Suzanne Somers: *Three’s Company and * Step by Step. Also She’s the Sherrif but that wasn’t a hit.
Patrick Duffy: Also on *Step by Step, Dallas, * and of course The Man From Atlantis!. 
Staci Keanan: * My Two Dads* and again *Step by Step *.
Does Bob Saget count for Full House and America’s Funniest Home Videos?
John Stamos was Blackie(?) on a soap opera (General Hospital?) and also on Full House.
Larry Hagman, “I dream of Genie” and “Dallas”
Patrick Duffy, “The Man from Atlantis” and “Dallas”
Adding to some names:
Kate Mullgrew was Mary Ryan Finelli on Ryan’s Hope. Yasmin Bleeth was her daughter Ryan Finelli and of course was on Baywatch.David Hasselhoff was Snapper Foster on The Young and the Restless. Ted Danson was Tim Conway on Somerset.
Jerry Van Dyke on My Mother the Car. Howard Hesseman was also on One Day at A Time, playing Anne Romano’s second husband.
John Ritter played a recurring role as a preacher on the Walton’s, and also was on the short-lived Hooperman (which I loved).
And how could we forget Kelsey Grammer on Cheers/Fraiser and as Sideshow Bob.
Including the neighbor, the one who wrote the sayings on greeting cards, on Mork and Mindy
Tiffany Amber Thissen on Saved by the Bell and Beverly Hills 90210.
Some far-seeing doper started a similar thread in February. Name some actors who have starred in 3 successful TV series
Robart Conrad starred in Black Sheep Squadron and Wild Wild West. He also did Search and Rescue (or something like that), and a few other things that were not nearly as successful.
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Connie Sellecca was in both “The Greatest American Hero” as Pam “Counselor” Davidson (later Hinkley) and “Hotel” as Christine Francis.
William Katt starred in “The Greatest American Hero” and later had a recurring role (Paul Drake, Jr.) in the sequence of Perry Mason telemovies.
Bruce Boxleitner, in addition to “Scarecrow and Mrs. King” and “Babylon 5” also was Luke Macahan in the TV miniseries and short series “How the West was Won”. The latter doesn’t quite qualify him for having 3 different roles in 3 different series (as per the link bibliophage posted above) but it was on in various forms from 1976 (as “The Macahans”)-1979
(IMdB is my friend)
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