I just went back to the classics, most of which are all free now to play online. the original Quake 3 Arena you can play on pretty much every OS for free now via ioquake3 and there are still a ton of servers for it. the original Unreal Tournament and Unreal Tournament 2004 are both available for free online play now and very easy to set up. I also prefer playing TFC over TF2. also the original HL Deathmatch is still a blast.

I THINK there’s also a free option for Tribes 2 out there but I’m not 100% sure on that.