Usually some singleplayer FPS with (practically) zero pauses for cutscenes/story/etc with quicksave+quickload so i can run it, play for 5 minutes, save, exit and pick it up at any time without having to wonder where i am or what is going on with the story. Pretty much always this is games i've already played at the past at least once. Recently it is Unreal 1 or Doom 3.
Also whenever i listen to some podcast (or video with just people talking) i like to boot up such a game (i often play Doom 3 BFG Edition since it supports gamepad and i can relax on my chair instead of hunching over the keyboard and mouse - i also use RBDoom3 sourceport which allows skipping cutscenes, etc and works natively on Linux) with the game's audio volume to barely audible.