The Danza: All cast and characters except Jason, because Jon Benjamin already plays John McGuirk, with Paula becoming a later exception (her UPN run being voiced by Paula Poundstone, her Adult Swim run being voiced by Janine Ditullo).
Brendon Small is an "As Himself except younger" example. Melissa Robbins is played by Melissa Bardin Galsky. Mr. Lynch is played by Ron Lynch.
Hey, It's That Voice!: H. Jon Benjamin, currently on both FX's Archer, and Fox's Bob's Burgers, is Coach McGurik (his voice for all three characters doesn't stray too far from his normal speaking voice).
Benjamin also plays Brandon's friend Jason. He uses a very similar voice for Jimmy Pesto, Jr.
Comedian Louis C.K. is the voice of Brendon's father.
The Other Darrin: Paula. Played by Paula Poundstone in the first 5 episodes (the ones that aired on UPN). Then, when the show was Un-Cancelled and put on Cartoon Network, she was played by Janine Ditullio for the remainder of the series.