What are some sleeper hit games that you enjoy?

A few that come to mind for me are

• ASCII Sector - space roguelike that was released in the last 2000s
• Sonic Robo Blast 2 - free 3d sonic fangame, built using a Doom engine of all things...
• ClassiCube - free sandbox block-placing game that is similar to 2009/2010 Minecraft Classic
• Chronicles of Denzar - free browser-based RPG similar to dragon quest (previously PhantasyRPG.com) running since the early 2000's