Ryan Cartwright, star of the Grimleys and Seriously Weird, among other things. Great actor and very funny.
Find yourself a showing of the current film New Suit. One of Dan Hedaya’s best performances.
I have a ton of actors that I love that aren’t big names. I’m not sure whether they qualify as character actors or not. Some of my favorites: Sam Behrens, John Bolger, Charlotte Ross (no longer unknown because of NYPD Blue), Peter Wingfield, Eric Pierpoint, Suzi Plakson, Delroy Lindo, Cieran Hinds…I know I’ll think of more the moment I hit submit.
But my number one favorite in this category, if he qualifies, is Richard E. Grant.
I’ve always liked the least well known sibling in the Cusack family, Ann, who is probably best known as Shirley Baker in A League of Their Own, the Peach who couldn’t read her name on the team roster.
I’m also a big fan of Shia LeBeouf – if you don’t know him now, you will soon.
Could you expand on that just a tad, GL?
Luis Guzman. He shows up all over the place as a character actor, most memorably in “Traffic” and “Magnolia.” He’s starring in a TV series this year, but it sounds horrible – he’s not a comic actor.
Pete Postlethwait – best know as Mr. Kobiyashi in “The Usual Suspects,” but with fine roles in “Brassed Off,” “Dragonheart,” and “Romeo + Juliet.”
Colm Feore. Probably best known as Andre Linoge in the TV miniseries “Storm of the Century”, but also functioned as “that guy” in a number of good films, including “Chicago”.
Charlotte Ayanna
In 1993 she creamed the competition winning Miss Teen USA - Her first and only pageant. She was a true poster child, giving inspiration to foster children all across America. Her story is a true American dream. You can read about it in her out of print book Lost in the System.
Closest to having a big break: Being Ricky Martin’s love interest in his video, “She’s All I Ever Had.”
She’s been in a few movies, and could be huge, if she could only get that one break. If there is one person that deserve lots of success it’s Charlotte Ayanna.
Not to mention his fabulous work as Glenn Gould in Thirty Two Short Films About Glenn Gould.
Brad Dourif’s absolute bestest best role is as the baddie in Myst:Exile. If you solve the last puzzle wrong he comes and smashes your face with a hammer.
fizzy, I love you. You always manage to turn a “feel-good” type post into something gruesome by the last word.
For more of those beneath-the-radar actors, check out Fametracker.
Actually, he’s a terrific comic actor–his great funny performances outnumber his great dramatic ones. However, he may not be an experienced sitcom actor.
Mine? Of modern actors, I’d go with David Strathairn. I’m sure everyone would recognize his face, but he blends in seamlessly into his character roles, so few (aside from John Sayles junkies) quickly recognize him.
Also, some of the old-timers: Jimmy Conlin, Al Bridge, Raymond Walburn, Esther Howard, & Porter Hall (I’m a Preston Sturges fan, if you didn’t notice)
And as for leading ladies: Kay Francis (sigh).
Michael Kitchen, who’s been in Foyle’s War on PBS, Mrs. Dalloway, The Railway Children, and the recent James Bond movies. He’s an amazingly subtle actor with a warm, intelligent face that I love. I want him to be my dad. 
I love Strathaim. When I first heard that they were making a movie of Delores Claiborne, he was my immediate choice for Joe St George. And I wasn’t disappointed. I never am with his work.
One of my favorite character actors is Michael Jeter. Most may remember him from a TV show called Evening Shade, but I remember him from several movies and mini-series including Jurassic Park III and Tales of the City.
Also I do love Chloe Webb. She brings such strength and vulnerability to the role of Nancy Spungeon in Sid and Nancy. She was great in China Beach too. Have seen her in several other movies and mini- series, never playing the same type of role.
Tim Roth is becoming more famous, but I still think he’s relatively unknown. The Legend of 1900 stands out as one of my favorite movies of all time. But you can’t forget him in the diner in Pulp Fiction. He’s done lots of other stuff, but he’s very quiet about it.
Tim Roth was superb in Four Rooms.
Wally Shawn
He was “that guy” in The Princess Bride and a bunch of other movies.
Obscure, but gaining ground, are Sam Rockwell
(“The Green Mile”, “Confessions of a Dangerous Mind”, “Matchstick Men”)
and William Mopather (who is Tom Cruise’s cousin) who has done
“In the Bedroom”, “Mission Impossible II” and “Minority Report”.
My favorite actor that nobody’s heard of is Mark Lindsay Chapman. He hasn’t been doing much lately (as far as I know), but probably his most visible roles were Dr. Arcane in the old “Swamp Thing” TV series back in the early 90s and Nick Hopewell in the TV adaptation of Stephen King’s “The Langoliers.” He might be most famous for losing the role of John Lennon in a movie in the '80s because Yoko Ono didn’t like his name.
Oh, and I second Rufus Sewell. Loved him in “A Knight’s Tale,” “Bless the Child,” and “Dark City.”
In this particular case, I can’t say I blame Yoko.