Free games :
osu! (of course), Spelunky, Cave Story, Momodora 2, almost every visual novel (Katawa Shoujo, Clannad to quote a few).
Not-free games :
Touhou (except 13.5/14), Terraria, Starcraft (the first), Warcraft 2/3, Age of Mythology, Age of Empires 1/2, Counter Strike Source (in fact, almost everything based on Source except DotA and Team Fortress), Starbound.
osu! (of course), Spelunky, Cave Story, Momodora 2, almost every visual novel (Katawa Shoujo, Clannad to quote a few).
Not-free games :
Touhou (except 13.5/14), Terraria, Starcraft (the first), Warcraft 2/3, Age of Mythology, Age of Empires 1/2, Counter Strike Source (in fact, almost everything based on Source except DotA and Team Fortress), Starbound.