I've been playing both shovel knight and enter the gungeon recently. I noticed in both games that there were crossovers / cameos in them (god of war and battletoads in shovel knight, and payday 2 in enter the gungeon) and love seeing that happen. Then I watched a super meat boy run and saw indie characters in that too.
Was just wondering. What games have you loved with characters special guesting in others (not including smash)? Playable, NPC, whatever.