The Gears of War protagonists appear in Lost Planet 2.
Albert Wesker from Resident Evil is a skin in Lost Planet 2.
Resident Evil characters appear in Project X Zone, alongside Megaman.
Megaman appears in Super Smash Bros with Samus,
Samus and Ridley appear in the background of a stage in Dead or Alive, which features a playable spartan from Halo.
Master Chief’s armor can be found in Duke Nukem Forever.
Duke Nukem Forever features a “Dead Space Marine.”