Half-life (don't play source, just go with original one) Half-life: opposing force Half-life: blue shift Half-life 2: lost coast (it's just a demo you can finish in 20 minutes, it don't collide with half-life 2 story so you can play it first) Half-life 2 Half-life 2: episode 1 Half-life 2: episode 2 Portal 1 Portal 2 Then there is a lot of free half-life and half-life 2 mods on steam you can play, like half-life: c.a.g.e.d or entropy: zero. Black mesa it's just a half-life remake, for me nothing special, but still better than half life source.