So, you are sick of World of Warcraft, of Battlefield 2 and Counterstrike Source? You want to experience something new? You've come to the right place my friend. I found a site that contains an extensive, and I mean extensive, list of many, and I mean many games that are free to play and concentrate on multiplayer experience. I don't think it's possible to test them all but you might just test all games that look interesting and fit the category of games that you like most.
There are many great looking games among the list, for example the game A3 has stunning visuals and is relatively unknown. You also find the usual suspects in the list, like Americas Army and Anarchy Online: Still, most games are unknown to the majority and many are worth a try. If you are looking for a new experience give this site a try.
Update: The list has not been updated since 2008, which means that it will list lots of games that are either not available anymore at all or games that have not been updated by its development team for some time. While you will find games in the list that you can still play online today, the majority are quite old looking. While that should not deter you right away, you may want to use a resource instead that is providing you with updated information about free to play MMO games.
Resources that you may want to check out are What MMORPG, with its updated game listing sorted into categories like strategy and kids mmorpgs, Free MMORPG List which lists all games from latest to oldest on multiple pages on its site, and MMOHut which lists games in categories like Sci-Fi, 2D or Racing. These three resources are regularly updated, and should provide you with up to date information and links to the latest and greatest in free MMOs.
Ghacks is a technology news blog that was founded in 2005 by Martin Brinkmann. It has since then become one of the most popular tech news sites on the Internet with five authors and regular contributions from freelance writers.