MetaCritic

Compiles every review on games, films, music and tv and comes up with a meta score for it.  Also has links to all the individual reviews.  a bit of a hassle sometimes to look though as there are so many but equally pretty well every review out there will be listed

Gamespot

Got a bit comercialised a few years back but still a pretty good middle of the road review site, reviews most games and gives fairly reasonable scores.

Firing Squad

While it has a few reviews the main reason to come here is for news on games.  They'll often have screenshots, and trailers for up and coming games, particularly if there's just been some big gaming conference on (e.g. E3) and all the game developers are showing off their latest work.

  Bit-tech

This site is quite good because it makes a special effort to review some of the better indy games most other sites forget (e.g. they reviewed mine craft before most of the world had heard of it).

 Co-Optimus

Good site for multi-player gaming, particularly co-op.  Great for looking for games to play together.