Personally I've sunk the last year and a half into a free-to-play called Warframe. You take the basic play style of Mass Effect 3 Multiplayer, throw in over a dozen mission types, randomly generated maps using over 20 tile sets, 24 different characters (and 10 deluxe "Prime" versions with slightly improved stats), over 200 weapons, over 500 mods to collect to customize your loadout, and then add in an amped up version of Mirror's Edge's free running. There's even outer space missions using flight packs called Archwings. It's still technically in Open Beta even after two years because the Dev team is constantly updating and tweaking the game and adding in new content. It's completely free-to-play, the only thing that can't be earned in game is cosmetic items, and even then you can farm for rare items and trade them to other players for the premium currency.
-------- Wedge Defense Force General Order 12: "Try to avoid freaking the mundanes."