If you're looking for professional reviews on the web, you'll have to pay a subscription fee, but here are some of the most popular...
Robert Parker (The Wine Advocate) is controversial because he rates wines on a 100 point basis, is followed widely by both drinkers and makers of wine, his ratings most influence the movement of the price of wine and some believe wine makers all over the world have figured out Parker's palate and have crafted their wines accordingly. While I think there is some value in the criticism Parker receives, I also believe some of the criticism is from people who resent his success and want to lay every problem in the wine world at his doorstep because he's a convenient scapegoat. Love him or not, he's a force in the wine world and it doesn't hurt to be aware of his reviews and ratings. If your palate matches his, you in bidness!
http://www.erobertparker.com
The other big one one is Wine Spectator. Spectator is controversial, too. And successful. Parker publishes his reviews in a newsletter and does not sell any advertising. Spectator does. They also use a 100 point rating system. Just about any time you see a rating attached to a wine, it will be either from Parker or Wine Spectator. Both Parker and Spectator also publish lots of other stuff on their sites, informative articles on wine, trends, wine makers, etc.
http://www.winespectator.com/
Next up is Steven Tanzer (Wine Access, International Wine Cellar), former Senior Editor and wine columnist for Food & Wine magazine who wrote Food & Wine's Official Wine Guide in 1998 and 1999. Tanzer also uses a 100 point rating system, but tends to be a little tougher than either Parker or Spectator. He is not as visible or successful and -- surprise -- not as controversial. He and the other reviewers on his site are pretty reliable and you will see some wine sites attaching Tanzer's scores to their wines, too, if they have any room left after posting Parker (Wine Advocate's) or Spectators.
http://www.wineaccess.com/expert/tanzer/newhome.html
Jancis Robinson's uses a 20 point system and is a well respected internationally known wine critic who often brings a different perspective....
http://www.jancisrobinson.com/index.php