This is my fav chess web-site!
Tons of players from all over the world, whose level varies extremely: from amatorial to professional!!
Not only online matches, but also correspondence chess online, training, etc… and all for free!
You can also become a member, pay a fee and get some extra service. Personally, I have very nice time without having bought the membership.
If you want to improve and learn new skills, you should consult the study plan directory