Brian Williams seems pretty close to just having a second career as a comedian. Aside from being on Stewart/Colbert frequently, he’s on 30 Rock at least once a season, has a regular bit on the Jimmy Fallon show, and is a pretty frequent guest on the other Late Night comedy shows. I think he’s done Wait Wait Don’t Tell Me more then once as well.
Agree he’s pretty good at it though.
Jesse Ventura, from his days of a WWF commentator to his political run, he is damn funny.
Richard Feynman, anyone that hasn’t read his “Surely you’re Joking Mr Feynman” needs to now.
David Lee Roth, treads the line of incomprehensible insanity very well.
These are very funny. The plane crash one’s a bit dark, but I like it. 
Ian Holloway, an English soccer manager, is renowned for his words of wisdom:
Look him up on YouTube too - the Bristol accent makes him all the funnier.
Where do y’all put Karl Pilkington? If he is a comedian, he’s a kind of anti-comedian.
NSFW for swears and a yogi showing his junk
Jim Bouton, former Yankee pitcher, wrote the funniest book I’ve ever read, “Ball Four (Five).”
The two funniest people ever who were not paid to be funny are Dorothy Parker, a writer, poet, critic and member of the Algonquin Round Table and Oscar Levant, a classical pianist who also wrote two (?) hilarious memoirs.
David Niven wrote some very funny memoirs as did Charles Grodin.