During the show actual (real) people were mentioned. I believe there is a list somewhere but I can't find it. So under this topic list the real person and the episode if you know it.
I'll start with Glenn Ford in "The Bank Job"
I know there are lots of others.
In "Barney's Sidecar" Barney mentions Alexander Graham Bell and says "People laughed at him right over the telephone"
Also Wilbur and Orville: ..they laugh them off Wright Air Force base.
In the Season 6 Episode 10 they mention all kinds of Hollywood stars. Doris Day, Dean Martin, Jack Benny, and Caesar Romero. Aunt Bee brags about her game show winnings.
In Earnest T. Joins the Army episode, Barney wants to throw a dragnet over the whole town to recapture E.T. Bass. Andy tells him they're not after Matt Dillinger.
In the episode "The Song Festers" Elanora Poultice mentions Peter, Paul and Mary when she is trying to think of a catchy name for Andy, Barney and Gomer singing trio.
"How about Taylor, Fife and Pyle?.... No... That sounds too much like Peter, Paul and Mary"
Anybody mention Marconi, Barney's inspiration for his high-tech listening devices in "The Great Filling Station Robbery"?