http://www.firefallthegame.com/
I'm going to be playing this a bit now and then. It's a free to play, Open World, FPS MMO. It is made up of on the fly PvE quests that spring up in the world around you.
There's also a deep crafting system that takes material quality into consideration during the construction process. You gather materials by using a, "thumper," to mine the resources from the world. The process of thumping draws random critters which try to stop the thumper. Thumpers come in different sizes, including special, "squad sized," thumpers for thumping large amounts of material as a team. The bigger the thumper, more/harder stuff comes to stop it.
The game is completely free to play. There are things that you can buy to give you a bit of a time investment edge such as faster XP, a vehicle, etc, but it's not a pay to win system.
http://www.firefallthegame.com/