More and more, video game developers are adapting to a Free to Play(F2P) model for multiplayer games.  This model has been so successful that some games have switched to this model, and then became even more successful. This benefits you, the gamer, by providing you with hours of online entertainment for free.   In addition, paying $60 just to try out a new game doesn't seem like a good use of your hard-earned money.  Free to Play games allow you to try out a game, and decide if its worth more of your time and money.  Let's take a look at three free to play games available today.  Two that started as F2P, and one that is transitioning.
In League of legends, you play the role of a summoner.  As you gain in summoner levels, 30 in all, you gain a point to put in a skill tree.  That skill tree will benefit the champion you select at the beginning of a match.  You have the potential, over time, to summon and control over 100 different champions.  Each player gets to choose one champion at the beginning of each match.  Champions start at level one at the beginning of each match, so the only advantages are skill and experience.  If you own the champion, you could use the same champion over and over.  You could also change your champion every game if you wanted to.  You use currency earned in game, or bought with real money, to buy champions.  In addition, there are a select variety of champions available every week.  Each champion has its own special abilities and a different style of play.  In these matches, you team up with 4 other summoners, and go head to head against another team.  The first team to knock out the enemy teams nexus wins the match.  There are also several types of matches.  Some types require you to destroy towers, while another type may require you to capture towers.
World of Tanks is another game that started with a F2P model.  In World of Tanks, you outfit your choice of tank, and then charge into battle with other players in giant maps designed for tank versus tank warfare.  There are different types of tanks to choose from, each with its own advantages and disadvantages.  You unlock different tanks through experience, and then with a currency purchase.  Currency is gained through playing the game, or buying it with real world money.  The tanks are all from World War 2 era, and span several countries.  More tanks and maps are added all the time.  The gameplay is very simple and straightforward.  You can keep multiple tanks in your garage so that when you die in one match, you can use another tank to jump into a new one.
Star Wars: The Old Republic announced on July 31, 2012 that it was transitioning to a F2P model in the fall of 2012.  Star Wars is more of an MMORPG, or Massively Multiplayer Online Role Playing Game.  What this means is that instead of a series of matches, the game world is constant and persistent.  When you log off, the world is there still for thousands of other players to experience.  You level up by completing quests, PvP battles, space missions, or flashpoints.  The game features fully voiced cut scenes for nearly every quest.  Each character class also has a unique storyline of quests that changes based on your choices.  At endgame you have several things to do.  Instanced PvP battles, called warzones, where you battle other players for "objective points".  There are also flashpoints and Operations where you join groups of players and work to defeat difficult enemies in a short story type experience.  Some servers are gathering places for roleplaying.  This means you create a character, like in a TV show or movie, and then act it out as you would in a drama club.  There are several other things you can do as well from daily quests to item crafting.  Star Wars is a full package MMO.
These are just three of the big free games out there.  New Free to Play multiplayer games are released all the time by developers large and small.  Good quality Free to Play games can even be found in the Google Play and Apple mobile app stores.  A few upcoming Free to Play multiplayer games you might want to keep your eye on are Repopulation, Firefall, and End of Nations.  Meanwhile, these other three games should keep you occupied for some time.  While there are some Free-to-Play borders, there is still a lot of open territory.