Naked City and Route 66 had countless early appearances, with NC's location shooting in NYC offering plenty of actors their shots, such as Dustin Hoffman, Robert Redford (playing a neo-Nazi!), Gene Hackman, Alan Alda and Robert Duvall. Route 66 had all of those--except Hoffman.
In an early episode of George C. Scott's "East Side, West Side," (also shot in NYC), Alda appeared as a john in the first episode, while Carroll O'Coonor plays a very Archie Bunker-type father who wants his daughter's (his little goil) boyfriend prosecuted for statutory rape the following week. Cicely Tyson played Scott's secretary until she got written out halfway through the only season--reportedly because CBS didn't like it.
A Hazel episode had Dick Sargent playing the nephew of George Baxter's boss--who wants to get married...to a woman. It also had a Jamie Farr and James Doohan (Scotty from Star Trek) appearance. The one-season revamp (in 1965-66) had a teenage Ann Jillian as a recurring character.
Highway Patrol had Joe Flynn and Robert Conrad in early roles. I didn't notice Flynn at first because he was trying to play a tough guy (in the standard issue leather jacket) and didn't have his usual horn-rimmed glasses. However, that nasal voice was a dead giveaway.
Teri Garr and Rob Reiner appeared on Batman. Reiner also played a hippie on Gomer Pyle.
Manson family victim/actress Sharon Tate appeared on the Beverly Hillbillies. Fellow victim, hairdresser Jay Sebring, had a cameo in a Batman episode--because he did producer William Dozier's hair.
One of Charles Bronson's last TV show appearances was as a federal agent who stumbles across Richard KImble during the final season of The Fugitive.
Mike Farrell played an agent on The Monkees
Adam West plays a cold-blooded killer who tries to frame the Carwrights in a 1961 episode of Bonanza. The following year, Gena Rowlands played a prostitute who Hoss wants to marry.