I've just voted (1), and not without reason.
Many games (especially of the RPG genre) provide lots of excellent material. Some can actually be pretty difficult for a beginner, if no translation in one form or another is included in the version published in a given non-English speaking country. Most of the games launched here in Poland are published as the so-called 'movie' version, i.e. with original dialogues and subtitles in Polish, but there have been many with no translation whatsoever, coming with just a simple installation / keyboard control manual. The latter can pose substantial difficulties (the excellent Ultima series being a good example), and I personally know more than one teenager who decided to take English classes more seriously, or even do some studying on their own, because of the game they were exploring.
Since the time I caught interest in military aviation simulators, my range of related vocabulary has expanded enormously. I wouldn't have gained so much from any formal course, I daresay.
An interesting topic in its own. Let's wait for more users' input.