Super Mario Crossover was voted the best free PC game of all time by GamePro.com, so I investigated in the name of investigation and ponies. (Everything I do, I do for ponies.)
From the outset, Super Mario Crossover was an impressive affair. There were options, options beyond your wildest dreams. You can play as Mario (who surely needs no introduction or explanation, Link (Zelda), Mega Man, Samus Aran (Metroid), Simon Belmont (Castlevania), Bill Rizer (Contra), Sophia III (Blaster Master) , and Ryu Hayabusa (Ninja Gaiden).