NetHack is an open source turn-based ASCII RPG that has been in development from 1985 to 2003. The game has randomly generated dungeons and an incredible variety of monsters, items and classes. Everything interacts with everything else, in too many ways to mention. If you can think of a way to combine items, the devteam almost certainly has thought of it as well. There are many ways to deal with any situation, but there are also many different dangerous situations. This makes it an incredibly deep and interesting game. It is one of my favourite games of all time.
Claw is a very difficult and very fun abandonware platform game. It requires insane timing at some points, and it was one of the few games I never managed to finish. Frustrating, but fun. Not freeware, but it's in IP hell and will never be sold.
Startopia is a humorous game where you develop an abandoned space station. The station is visited by 9 alien species, all with their own tastes and specialties. These are your workers as well as your costumers. The station is divided into three specialized decks, and your costumers need all three of them. The game is made by former employees of the legendary Bullfrog Games, who left after that developer WAS FUCKING DESTROYED BY EA BASTARDS BASTARDS BASTARDS BASTARDS!
The game is now seven bucks on Steam.
Fall from Heaven II is not a standalone game, but a mod for Civ IV that turns it into a very well realized fantasy world. It combines the strong mechanics of Civilization with the excellent worldbuilding of its designer, Kael. The whole thing is brilliantly done - there are 21 civilizations, all with a unique theme and playstyle. There are golem-building dwarfs, pirates, ultraviolent savages, evil clowns, gods-hating humans and many more. I spent more time playing this mod than normal Civ4. This is also one of my favourite games.