Path of Exile, a Free to play ARPG long in the making what just came out on Steam. It's pretty fun and well-made, with a shit-ton of possible builds. It's less a skill tree than a skill forest, honestly, albeit all of it is passives, most of them boring (+5% sword damage!). Actual skills use a more interesting system, being tied to gems you socket in weapons.
To say the game is, otherwise, a throwback to Diablo 2 is putting it lightly. It's not a copy-paste, but it's very clear what game the devs are basing themselves on, hell even the minimap looks almost identical, and the itemization is almost copied word for word.
Still, lots of fun is to be had, and it looks pretty damn good to boot, especially for a F2P indie game.