http://en.wikipedia.org/wiki/Defense_of_the_Ancients
A band (Swedish I think) wrote a song inspired by it lol
Still completely free (unless you buy riot points for skins or special champion packages, but that is totally optional.
I don't know how many other people play it, so that's why I made the forum because I wanna get some =F|A= on my friends list for it if they play.
I'm currently level 17 (out of 30) in the game. it is an entirely PvP (player vs player) game that pits groups of people called "summoners" who select "champions" in the waiting lobby of the game. Each champion has 4 abilities (some have forms that grant additional abilities) and 1 passive ability. Each champion can progress to level 18 throughout the course of the game. You acquire gold throughout the match to purchase items by killing enemy champions, killing minions (trash mobs), destroying turrets or inhibitors. You can get bonus points if you kill someone who is on a killstreak and based on the size of said killstreak it will award an amount of bonus gold accordingly. Throughout the 18 levels of the game, you get 1 point to spend per level. This point can be spent to increase the rank of 1 of your 4 abilities. The first 3 abilities are available at the beginning of the game and each of those abilities has 5 ranks, increasing damage/healing, reducing/increasing mana cost, decreasing cooldown. The 4th ability is your Ultimate ability which is an ability that defines that specific character, and that ability has 3 ranks, each rank exhibiting the same pattern as with your previous 3 abilities as you upgrade. The game is won by defeating the enemy "Nexus" which is located in the center of their base. There are several "lanes" that lead to the enemy base (2 lanes in the 3v3 map and 3 lanes in the 5v5 map). Along each lane are several turrets, which have a large amount of health and deal significant damage and must be destroyed before progressing down the lane. To compensate for the significant damage output of these turrets and to prevent champions from consistently dying trying to defeat the turrets, they have minions. Minions are fairly low health creatures that are AI controlled that will attack the enemy turrets/inhibitors/nexus, attack enemy minions, and attack enemy champions. Getting a killing blow on one of these minions awards an amount of gold, that adds to your gold stores to buy items to increase the potency of your champion. Once one of the turrets in question is destroyed, you can progress farther down lane to destroy another turret, eventually reaching the enemy inhibitor which allows you to attack the nexus, and grants your team stronger minions. Once the enemy Nexus is destroyed, you win the game, but if your nexus is destroyed, you lose the game.
I think I covered most of everything, its a really fun game once you get into it lol.
/end wall of text