Games where a person from real life is a playable character.
For games where you play as a fictionalised version of a real person, where the 'fictionalised' version has become a character in and of itself.
Professional and College Sports Players, Sports Games Musicians in the Guitar Hero and Rock Band series
Larry Bird & Julius Irving (Dr. J and Larry Bird Go One on One) Daley Thompson (Daley Thomphson's Decathlon) Emilio Butragueno (Emilio Butragueno Futbol) Eddie Edwards (Eddie Edwards' Super Ski) Emilio Sanchez (Emilio Sanchez Vicario Grand Slam) Eric Bristow (Eric Bristow Pro-Darts) Eddie Kidd (Eddie Kidd Jump Challenge) Angel Nieto (Angel Nieto Pole 500) Barry McGuigan (Barry McGuigan World Championship Boxing) Carlos Sainz (Carlos Sainz: Rally Championship 1990) Fernando Martin (Fernando Martin Basket Master) Gary Lineker (Gary Lineker's Superskills) Brian Jacks (Brian Jacks Superstar Challenge)
geoff capes (geoff capes strongman) david beckham (go! go! beckham!)
Tomonobu Itagaki is a preset character choice in Saints Row: The Third.
MMA fighter and Japanese internet meme Bobby Olugun is playable in the PS2 revision of Power Instinct Matrimelee. I believe he was removed for the later arcade revision.
3LW is an unlockable team in the first NBA Street.
Gameloft made a pathetic cellphone game to cash in on the Chuck Norris meme. Also there was this, featuring Chuck Norris as I think himself on the Collecovision:
Lagaf' in Lagaf': Les Aventures de Moktar, later re-edited as Titus the Fox.
The Beastie Boys are an unlockable team in NBA Street V3 and NBA Jam: On Fire edition.
Shaquille O' Neal and Micheal Jackson were also unlockable in Ready 2 Rumble Round 2
The Tony Hawk games have had a lot of non-skaters as secret characters including
Billie Joe Armstrong, Lil' Jon, Gene Simmons (As mentioned in the OP), Steve-O, Cut Chemist and Travis Barker.
Xenosaga: Mary Magdalene and Jesus
Henry Morton Stanley in Stanley : The search for Dr Livingston.
Shadow Hearts: Mata Hari
Shinya Arino in Retro Game Challenge
Danica Patrick in Sonic & Sega All Star Racing Transformed
There must be a bunch in historical strategy games. All the Three Kingdoms games, etc–though sometimes you are not nessesarily playing as any of the characters, I guess. There's that Napoleon game on GBA (that I would love to try some day) where you control him directly on the battlefield. There are the Genghis Khan games. There must be some Roman and Greek ones. You can be Oda Nobunaga in the Nobunaga's Ambition games.