BoardGameGeek

Appropriately, this is the first site review entry here.
This is the ultimate site for anything to do with boardgames, with a massive user-contributed catalogue of probably every game that's ever been available.

The first place to go to find info about a specific game, rules, variants, reviews, etc. The unique community user-ranking mechanism is a valuable way to get a summary of how well a particular game is regarded by the gaming community at large.

User participation is free, and it's easy and fascinating to surf through the collective and individual personalities of those that contribute to this site.

BoardGameGeek