IMHO, the only games you could call original would be things like Pong and Asteroids - ie. the very first electronic interactive arcade games.
Given that technology has been evolving at a steady pace, its natural that games evolve with this technology - and when you look at just about ANY game it has stemmed from a title before it with perhaps one or two new features.
So, really, there are hardly any games that are truly original in the sense of the word, they've just taken inspiration from older games and built on them with a little more technological flexibility.
Obviously the only things that are "original" are the new features - ie. the changeover from 2D graphics to 3D graphics a la Wolfenstein, the changeover to point-and-click adventures from text-based adventures etc. etc... games are like any other art form - 90% inspiration 10% perspiration...
(well, actually it's more like 90% inspiration 90% perspiration, since programmers have to put in 180% effort to make a bloody game
... pesky bugs, how dare they infiltrate my perfect code)
The one good thing (and bad thing - depends which way you look at it) about games these days is their development time. A decent game can take up to 18 months to make (I believe Gran Turismo 3 took around this long) which (hopefully) means that developers looking for a quick buck will be deterred - there's nothing worse than cheap clone games that offer nothing new and are just looking for a quick buck. I suppose the obvious flaw to this argument is 18 months of development time can still produce a pretty run-of-the-mill game. A crap game with 100 vehicles to choose from and 50 levels isn't much use to anyone if it sucks :-)