In a totally unexpected move, Interplay are about to re-release Planescape: Torment, which has an excellent claim on being the single greatest computer roleplaying game ever made. Easily the closest gaming has ever gotten to 'real' literature, Torment is a complex and morally ambivalent game about an enigmatic central character who cannot die and has to uncover his past. The setting is the wondrous Planescape multiverse from 2nd Edition Dungeons and Dragons (look out for its eventual appearance in the 'Worlds of D&D' feature), the most original setting that D&D has ever produced, where ideas and ideology can shape the very nature of reality. The game is notable for not focusing on combat and putting the characters and their thoughts, conflicts and ideas front and centre, with over 400,000 words of dialogue and description making up the game.
Torment was produced by Black Isle by much of the same team that worked on the first two Fallout games and would go on to make the Icewind Dale duology before going bust and reforming as Obsidian, creators of Neverwinter Nights II and Knights of the Old Republic II. Originally released in 1999, the game was overlooked somewhat by many gamers at the time (although it went on to sell a solid half-million copies) but has become a major critical darling in recent years, frequently placing high in 'Best Game Ever' polls. This strategic, Vista and Windows 7-compatible re-release will hopefully introduce it to a new generation of gamers.
The most eyebrow-raising part of this is the price: a ten-year-old game for £17.99? Seems a bit much. I'd be tempted to say hang on for a while and see if it drops below a tenner, but then again it is seriously one of the very best computer games of any genre ever made, so at that price it is still reasonable, if you can withstand the old-school graphics (and there are plenty of mods out there which can improve the resolution and interface).
Simultaneously, you can also get the almost-as-awesome Anachronox from Amazon for £4, which is a total no-brainer.