I'd agree with a good number of suggestions in this thread but avoiding repetition I'd say;
Live for Speed - If you're in to your sim racing then it is still up there for me as one of the best. Graphically it's old though you can get fan made high res textures which improves things without too much of a hit to FPS. Handling / physics models are a little subjective but I think LFS still has one of the best.
Modded Minecraft - I know Minecraft was suggested above but I'd add to it and say specifically Minecraft with mods. I don't mind vanilla MC but the mechanics are lacking which are where mods come in. Specifically I'd lean towards the 'kitchen sink' mod packs like the FTB ones as they are (mostly) cohesive packs but there's loads out there. They add machine complexity, automation, magic, more mobs/bosses, rpg elements, story driven worlds etc. The one I'm looking forward to is TangoTek's villager mod if/when it gets released as it will involve building a working village, adding villagers who will to do their specific tasks - farm, chop wood, mining, blacksmithing etc. so it's more strategy-esque.