Dad’s Home! - Safe for work, provided you have headphones and can keep yourself from laughing. No real offensive content.
MC Hawking - A Brief History of Rhyme - Some swearing, hosted on Newgrounds.
That’s all I can think of right now.
MC Hawking has many mp3’s on his website, which includes a discography, and other goodies, the last time I saw it.
I have never quite recovered from the Kikko Man.
There is one Flash animation that I’ve had in my favorites for about 4 years now. I’m at work and don’t have the link, but I’ll post it after I get home at 5 EST, unless someone beats me to it.
That dad’s a real hot head.
Was that not music from F-Zero playing at the end? Woo!
I presume most of you know about albinoblacksheep.com, which has loads of Flash movies. Some of my favorites (all of these have sound but are work-safe unless noted otherwise):
Fuzzy Math
Mashed Taters
Milk and Cereal
Mario Twins (has profanity)
Cereal Bawks
Maiyahi
And on a different site:
Salad Fingers (weird stuff, like impaling fingers on hooks, and possibly language as well - I’m not positive)
And of course everyone knows Homestar Runner.
Gr8 thread! Now I don’t have to rely completely on co-workers to tell me about sites like big-boys or ebaumsworld.com. The video I cackle at most like a hyenia has got to be this farked Chinese tree catching disaster
Business communication. Needs headphones, vaguely safe for work.
Waterman is very funny. It reminds me somewhat of Family Guy.
Resident Evil: Flash Edition is friggin hilarious. And I’ve never even played Resident Evil.
Optical Illusion: http://www.laserp.com/fun_stuff/illusion.htm
Punch the Boss: http://www.laserp.com/boss.htm
Stupid Videos: http://www.stupidvideos.com/Default.asp
Amy Winfrey also does:
Big Bunny
Muffinfilms
Enjoy!
There also Kubrick2001, which goes through the movie 2001 in about 10 minutes. It’s actually pretty good.
(Not Flash, but interesting, is The Picture Of Everything)
That’s so weird. On my first go I found Waldo.
And I wasn’t even trying.
That was great. I’ve read the book and seen the film, but I’ve never read any analyses of them. Makes the story even more wonderful. 
Somebody already listed albinoblacksheep, but just in case you don’t find it on your own: Irrational Exuberance.