http://www.game-revolution.com/content.htm This is probably my favorite review site based entirely on the quality of reviews - they are both well written and often funny without being stupid (i.e. no "potty humor"), a rare quality when it comes to game reviewers, most of whom couldn't write their way out of a paper bag, much less do so humorously.
BUT ... they are a small site and tend to cover a few things at a time, rather than hitting most of the new games out. http://gamesdomain.com/ Games domain is okay, not the greatest, but they won't insult your intelligence and hey - their site is totally free!
I regularly check Link Removed because along with their own reviews they post a daily round up of reviews on the net (not complete, by any means, but they do link to a lot) along with game related news.
After that it just comes down to system specific sites. I have an Xbox so that is what I go for. Even many of the pay sites post free reviews or eventualy make their pay reviews free, so I still check them out.
Happy hunting.
EDIT: Whoops. I just realized you were asking strictly for PC gaming sites. Oh well. The sites I listed DO post PC game reviews, along with consoles, but I know that wasn't what you were looking for. Sorry. (gamespy is probably your best bet since they do a round up of PC game reviews each day too.)