Most crossover characters in a platform videogame

Ultra-hard indie platform game Super Meat Boy (Team Meat, 2010) has been a huge hit on both PC and consoles and acted as an inspiration for independent games developers all over the world. In a heartwarming gesture of unity, a raft of developers agreed to have their characters featured in the game, resulting in 18 playable cameo appearances from different games series, including Half-Life (Valve Corporation, 1998), Braid (Number None, 2008), Alien Hominid (The Behemoth, 2004) and Minecraft (Mojang, 2011).