Video game crossovers

Sometimes when one video game loves another, they have crossover babies. Heroes appearing in other games(Dante in Viewtiful Joe, for example), can you guys think of how many have existed? Nintendo games are a given(Brawl, Party, Kart)
 
Here's one of the newer, and more surprising ones. Mr. Isaac Clarke in MySims Agents.

That surprising. Then again MySims and DeadSpace are under EA Games anyway. I can't think of any right now but I'll be back when I do.

in metal gear solid 3: snake eater, they put in the monkeys from ape escape.

Ninja Gaiden/DOA's Ryu Hayabashi is going to be in the forth-coming Dynasty Warriors game.  Source. That's more of a inter-publishing crossover though. 
 
I guess you could count Kratos in the current PSP Soul Caliber game or Master Chief, Link and the Tekken dude as well as the Sith apprentice in the other SC games.  But, again that was more of a marketing ploy.
 
Wasn't there a Megaman costume in Dead Rising?

Picture of Luigi through the window in the courtyard at Zelda's castle in Ocarina of time.

 Dante from Devil May Cry in SMT: Nocturne, in MGS4 you can get an Assassin's Creed costume, and Battletoads & Double Dragon 

Gnawty from Donkey Kong Country in the "Click Clock Wood" world of Banjo-Kazooie.  Also, in Banjo-Kazooie: Nuts & Bolts, there is a statue of Banjo and Kazooie holding up a Jiggy in Banjoland.  This statue was also a decoration you could buy for your garden in Viva Piñata from the store in the game.

DOOM Guy was in Tony Hawk 3 i think

" Picture of Luigi through the window in the courtyard at Zelda's castle in Ocarina of time. "