On the old “Dragnet” shows of the 60s and 70s, Jack Webb re-used actors all the time. There was one little, old, bespctacled lady who appeared in a dozen episodes.
MASH was pretty bad about this when it came to the “Korean” characters. The only specific example I can think of is the peddler who constructed a new clamp for them (same episode that Klinger loses Margaret’s ring); he showed up a few other times.
Thought of a few more…
On Beavis and Butthead the character of Stewart’s father appeared as a teacher once. Not just the same guy doing the voice but the exact same character.
There is one recurring character on the Simpsons which kind of fits this catagory. He doesn’t have a proper name that I know of; I guess he could be called “The Hapless Employee”. It’s a teenage boy with a high squeaky voice who is always put upon by Homer and others, and is working at a different place every time. Maybe it’s the same guy, and Homer just gets him fired all the time.
Didn’t The Love Boat and Fantasy Island have guest stars return as different characters?
Mako played what seemed like more Koreans than there are in Korea.
Continuing the Star Trek theme, Denise Crosby played Tasha Yar and Commander Seela. The same actor played Trelane and one of the smooth-headed-who-later-became-enturtled Klingons, I don’t remember which and I’m too lazy to go to startrek.com and look it up.
On Bablyon 5, Ed Wasser, who played Morden (no spoilers in saying he’s a bad guy–he’s obviously up to no good from the start) also played a tech in C&C in the pilot. The first time we rewatched the pilot, we were like, “Aaah! Watch out Takashima! It’s Morden!”
there was this cop show called Toma. It was supposed to be based on the fact that some undercover cop named Dave Toma was a master of disguise, and was a real guy, to boot. One other gimmick the show had, was that the real Dave Toma played a different minor character in each episode.
Does he count as an actor? I bet he had to join the union to do the show, so I say “yes.”
Was that the girl that Bud Bundy lost his virginity to? If so, than she was played by none other than Joey Lauren Adams–possibly one of the hottest actresses alive! In addition to those two appearances on Married, she also played yet another character in a spin-off called Top of the Heap. That show’s pilot was aired as an episode of Married with Children…So that makes three different characters in the same series!
The actor who played Lurch in the Addams Family series also played Thing.
Brian Thompson (Alien Bounty Hunter from the X-Files) played a vampire called “Luke” in the first two episodes of Buffy, and then played “The Judge” in two episodes a season later.
Elizabeth Montgomery played Samantha and her cousin in Bewitched…
Another Married With Children… Ted McGinley first played Peg’s alternate-reality husband in the It’s A Wonderful Life-parody Christmas episode before he permanently became Jefferson D’Arcy.
I love Married With Children.
Jon Lovitz guest starred on NewsRadio before replacing the late, great Phil Hartman.
Nobody will ever replace Phil Hartman. 
In Home & Away, the actor who played Luke had already played Dave, Ben-who-married-Carly’s army friend who stayed with him in the caravan when Ben first arrived and met Carly.
In Neighbours, the actor Darius Perkins who played Scott Robinson in the first series of Neighbours, before the show was cancelled and restarted starring Jason Donovan instead, turned up later as some psycho who tried to burn down Charlene’s caravan.
Very caravan-dwelling, those Australian soap stars.
Especially that no-talent ass-clown Jon Lovitz.
On Night Court, John Astin had a variety of one-shot roles before he got a regular role as Harry’s father.
In the first season of Barney Miller, Steve Landsberg played a priest who got arrested. In the second season, he became Detective Arthur Deitrich, one of my favorite TV characters.
dirty harry series:
the guy at the other end of the gun in the “do you feel lucky punk?” turns up in a later movie as his partner. Does that count?
The Local Indigenous Personnel on MAS*H have already been mentioned. Before his recurring role as Col. Flagg, Edward Winter had a single guest appearance.
Also on Babylon 5…
I think JMS’ mantra was “when in doubt, use Wayne Alexander.” Three roles that pop into my head were “The Inquisitor”, Lorien, and the Drazi “prisoner” in Intersections in Real Time.
Adams also played multiple roles in Kevin Smith’s series of five ViewAskewniverse films, appearing Gwen in Mallrats and as Alyssa in Chasing Amy. Ben Affleck, Jason Lee, and others also played multiple roles in those films.
–Cliffy