Well, this game was New To Me, and I thought it was worth a link.
Rom Check FailAbsolutely batshit insane freeware game where the player character, enemies, music and stage graphics change randomly every few seconds, cycling through classic console/arcade games.
Even the character and enemy movement change, which means occasionally you'll get destroyed thanks to the randomising, but occasionally you'll be unstoppably powerful. It's surprisingly fun. I laughed with joy when my character turned into Mario above a whole swarm of Gauntlet monsters, and I just jumped on all of 'em.