So this is not strictly Linux related (although this does work on Linux with a bit of work), but I have recently been absorbed into a freebie game called 'League of Legends'. Basically if anyone has played the absurdly popular Warcraft 3 mod 'Defense of the Ancients', you'll feel right at home playing LoL. It's essentially the same game with extra options for customization between games and a cash shop where you can *optionally* buy extra heroes, skins and plenty of other stuff.
Technically speaking you don't ever need to pay a cent to play this game as most items in the store can be purchased using in-game money that is earned by playing the game, however it is typically a lot quicker to simply drop $20-$30 on a character pack (20 characters + a bunch of extras). But for those of you gaming on a budget it is DEFINITELY possible to play this for free, you can even try new heroes each week for free as there are always 10 or so different 'free-to-try' heroes available each week.
This video, while not being from MY prestigious channel is in fact a good overview of the game and also gives newcomers some useful tips.
As I said, this game is playable on Linux with a bit of work and this user-made tutorial will do the trick. It's not perfect on Linux, but it is certainly playable and as Riot Games is working on a Mac edition, Wine compatibility will improve greatly in the coming months.
Keep an eye out for "synicalxx" (just synical was taken, hence the two xx's...), maybe someone out there can teach me a trick or two!