Anyway, almost the only game I play in spare times since years, is The Dark Mod, it's free and works fine in Linux, Windows and even in Mac (using PlayonMac).
It is, without a doubt one of the best free games you can find, apart with a friendly community for the support and a complete Wiki with step by step tutorials for creating own missions. A game to enjoy for many years. Walkthroughs of all missions in YT in the case if you get stuck in a mission.