I always like to toss out BDO and Warframe. Everything important you need can be earned over time through in game means.
In Warframe's case, you will need to use platinum, a paid currency, to increase inventory slots eventually, but you can earn platinum through trading rare mods/equipment/resources/etc.
Path of Exile is always constantly praised for being CS independent as well.
BDO you just grind for money and buy things like pets from the AH or be around for events and get em for free.
