Hm... this topic is a bit tough to categorize, because there are really very few games that the 'masses' (ie, the casual uninformed gamers) really latch on. Mostly these are the GTA's, Halos, and EA sports games. A couple of the games on this list already I wouldn't personally call overlooked because I've always, since the time they came out, heard great things about them as well as knowing many people who have it and play them. Granted, this is mostly from people who play and know games, but if we're mentioning every game that hasn't had awesomely great commercial success, this game is going to have nearly every good game every made on it.
Anyway, I don't really agree with Descent, Planescape: Torment, System Shock 2, or Tie Fighter being on this list. Those games are definitely considered some of the holy grails of gaming even if they aren't really massive commercial wins, and they definitely are not currently forgotten or overlooked IMO.