So for those who have never heard of it, I Have No Mouth and I Must Scream is a point-and-click adventure game that came out in 1995 and is based on a short story by Harlan Ellison.  
 
The premise is apocalyptic and brutal: humanity has been destroyed by nuclear warfare instigated by the sentient AIs of the Americans, Russians, and Chinese. Everyone is dead except for 5 individuals who have been kept alive for 109 years so that the Allied Mastercomputer, due to insanity and boredom, can torture them. He doesn't stop at physical torment, but also preys on their psychological weaknesses. However, the computer offers them a chance at redemption and potentially freedom if they play a "little game" he has devised for them. The setting/puzzles of the game reflect the psychological traumas of each character: Gorrister feels guilty for his wife having been institutionalised, Benny fought in Vietnam and was responsible for the deaths of some of the soldiers serving under him, Ellen is a traumatised rape victim, Nimdok is a Nazi doctor and also a Jew, and Ted is just paranoid and sleazy. In addition, the game has an interesting mechanic called the 'Spiritual Barometer' - similarly to the pthnisychological meter in Indigo Prophecy, if it drops too low it could potentially end in game over, but its increase represents the character having resolved the issues of his/her past.
 
Due to the way it skillfully uses dialogue, characterisation, mechanics, and the graphics/music to create a thematic whole, this is amongst the best if not the best adventure game I've ever played. Perhaps because it is an adaptation, the characters struggles feel more believable and their emotions less artificial. As one might guess from the summary of each character above, the game pushes into uncomfortable and dark territory that games barely if ever come close to broaching in present times.  For example, Heavy Rain squanders some of its emotional impact with questionable thematic elements (the incorporation of SF was a bad idea) and an awful twist, even though I appreciated many aspects of it. I would consider the Silent Hill series one of the few exceptions to the rule in uncompromisingly dealing with difficult subject matter, but other than that I cannot think of many other contemporary examples-increased swearing, sexual content, and gore is baditude rather than something that genuinely challenges us.
 
Not that all games need to be dark and difficult, but it seems that this kind of quirky, challenging game had its heyday in the 90s on the PC. My hypothesis would be it was easier to get away with these kinds of games before extreme regulation and scrutiny from cultural conservatives. Now we cannot even include the name 'Taliban' in a game for fear of someone being offended. Not to mention the Australian censorship of Left 4 Dead 2, which is a disgrace that weakens the game greatly. With these kinds of pressures, it's hard to imagine a game like I Have No Mouth and I Must Scream being made in 2010.