Abandonware Simulation games
Simulation is a niche type of games, they try to recreate real life situation on a computer. The best examples are flight simulators which where quite popular in the 90s. Several simulation games where made by fans when the cost to create a game was still low. Submarines, fishing, industry or car invention games where made but these themes are no longer exploited in the game industry.
In the Simulation genre, you may want to play 1000 Miglia, 177, 1830: Railroads & Robber Barons, 1869, 1942: The Pacific Air War. Those are just suggestions, you are welcome to download any game!