Recs.
Updated
Quake III Arena is a first person arena shooter that focuses on quick reflexes and fast paced action.
SpecsUpdate
Pros
Pro Has modern improved Open Source implementation
Released under GPL Quake III Arena was forked into ioquake3 project that aims to support the game engine on modern hardware with security improvements, new APIs, and even couple cool features. ioquake3 is completely compatible to play with people running latest official release of Quake 3 v1.32, it is easy to build for linux, mac, windows, and some esoteric operating systems. You still need to have some original files from official release to run the full game though.
Cons
Con No matchmaking
It has come to be a standard these days for online multiplayer game, especially competitive ones to have some kind of ranking system in place. Those rankings are often used to match players of similar skill level together for best experience. Quake III does not have this, best you can do is use server finder or seek for a community to regulate this issue on their own.