Ok, so UT99 has a ton of people on it, more than I've seen in years(server browser just said over 700 people!). Most are familiar with Quake Live and other Q3 based games, but UT is different. The weapons are all very original and have useful fire modes, and the movement is more grounded, less about jump pads and pogo jumping, more about dodging.
The game is available in the Unreal Anthology pack which can be had for $10-$20, but if you only want UT99, just let me know and we'll see what we can come up with.
It was best known for it's mod community, and there are still some very good, and very much played mods for it. One that comes to mind is called "Monster Hunt". It's a co-op game type that has you traveling from point A to point B killing all kinds of monsters, and usually a final battle at the end with a huge creature. Lots of people play it and it can be loads of fun.
It's a Windows game, but there is a linux patch. I run linux, but since it's a tiny distro(puppy linux) it lacks many of the libs that are usually included with Ubuntu, so I just play with Wine and it runs at %100 for me. And I'm sure it will run, probably even on the highest settings, on any computer today.