The world’s biggest chess server, with about 20,000 chess players every day, from all over the world is Playchess.com. The advantage of the large number of members is clear: there will always be a player available of your own playing strength. The site also offers to kibitz grandmaster games and follow world class tournaments as well as training lessons and simultaneous games against a grandmaster.
However, the site is not not free, but offers a possibility to try it free of charge and without any obligation. Membership is about 30 euros a year.