My favourite online game is called Renegade X.
Renegade X is 100% free. There are no purchasable DLCs or in-app purchases. It is not pay-to-win. It's a straight-up-free game with no strings attached, period.
It is a multiplayer game that merges the First Person Shooter and Real Time Strategy genres. There are two very different teams, and each team has a base. The object of the game is to destroy the enemy base while protecting your own. Each team has a commander and an economy.
The game has tanks, helicopters, superweapons (nukes, ion cannons, airstrikes, cruise missiles), and a ton of cool stuff. It is PC-only and it runs on Unreal Engine 3.
Renegade X doesn't have a huge community, but the 64 player server fills up everyday, and on weekends there are 100-200 players.
You can download Renegade X at http://www.renegade-x.com