Here are some good chess links:
http://www.chesscube.com/ : A good free website to play chess. A lot of tournaments everyday, even special ones called Warzones. In these, you play as many games as you can within a given time. The quicker you win, the most games you play.
http://chesstempo.com/ : A great website to train tactical skills. There are thousands of tactical puzzles, which have a rating. By subscribing for free, you get a rating and are given puzzles according to your strength.
http://www.chessclub.com/ : The most popular and best paying website. You get to play thousands of opponents, at any time control. Lots of IMs and GMs are playing there, including some of the top GMs, like Nakamura.
http://www.echecs-online.fr/ A small but nice website to play. It is a French website, which is rare on the Internet. That is not where you meet the strongest opponents, but some are pretty good.
http://gmsmirnov.com/#oid=1111_10 : A blog made by a strong GM, containing lots of free lessons.
These are my favorite chess websites, which I wanted to share with you. If you know any other good website, feel free to leave comments.