One of the best multiplayer PC strategy games out there, and it's FREE.
In a nutshell, you pick one of 40 Champions (think "WoW character with 6 abilities"), team up 5v5 or 3v3, and level up while attempting to overpower the enemy team and destroy their base before they destroy yours.
The Champions are extremely varied, very imaginatively designed and full of character with great voice acting. You may have to try out a few to decide on your "main", but as a few examples, you have Cho'Gath the demon from hell who devours enemies to make himself grow in health and size; Heimerdinger the mad scientist who can set up a defensive wall of mini turrets; Janna the floating sorceress who heals and releases tornadoes; and Shaco the insane jester who can stealth and create a copy of himself to confuse the enemy.
If you start playing LoL with no experience of DoTA (as I did) there's a chance you'll be quickly disillusioned by the unforgiving attitude of some players when you mess up. If you can find the nuggets of helpful advice hidden in their searing anger you'll quickly improve at the game and be relieved to look on as other unfortunate noobs become the unwelcome recipients of such tirades.
Once you've started getting decent at the game and you leave the matchup tier referred to as Noob Island you'll find the community is a lot more friendly and mature, but this being the internet there will unfortunately always be a certain number of mouthbreathers to deal with.
Visually, the developers were smart enough to overlook today's irrational obsession with multiplayer games having over-the-top high poly graphics in favor of having a client that runs smoothly on a wide range of hardware. They've done a great job within the limitations they set themselves and the graphics are vibrant and polished with very fluid and natural animations.
One of the features that makes LoL stand out from the pack is the Summoner system whereby you personally receive XP and IP after each game based on your performance, which lets you level up and buy runes to buff your champions. As you level, you also gain points to spend on your talent tree. This really adds to the "just one more game!" hook.
If you like intense multiplayer games that reward teamwork and quick strategic thinking, it's very likely you'll enjoy League of Legends. Yes, there are features that have not yet been implemented but the coming additions (shop, clans, ladders, maps) will just add to what is already a very entertaining and polished game.
Very highly recommended.
This introductory guide covers the basics of the game and gives advice on the main things you need to know: http://www.leagueoflegends.com/board/showthread.php?t=8024
Sidenote: Gamespot's review is unreasonable insofar as it's written from the perspective of someone who bought the game box without knowing what to expect, while only making a passing reference to the fact that you can play the entire game for free. I strongly doubt there are many people who would buy any game these days without knowing anything about it and I'd suggest that the majority of the people who have bought LoL so far were beta players who wanted to support the game's developers because they want to ensure it has a long life. Given that the reviewer concluded that the game is fun and addictive it would make sense to give it a much higher score than 6.0 based on the fact that you can play it without spending a cent. Isn't fun what games are about??