Xonotic is a fast-paced arena-style first-person shooter for Windows, Linux and Mac. The program is also open-source, surprisingly-- the graphics and features already outperform some commercial products.
The game is convenient to play as there's no setup required. Just unzip the 945MB archive, run xonotic.exe and it fires up within seconds.
Xonontic can take a while to master. It has nine core weapons, 16 full weapons, each with their own primary and secondary fire modes and all useful in specific situations. And although you might think you'll have time to experiment, the gameplay is so fast that you'll have plenty to do in just staying alive.
Fortunately, there's a simple single-player mode available where you can practice with some bots. Once you're ready, go online and you can play against others in 25 official maps or dozens of community offerings (including Nexuiz and conversions from Quake 3), with a huge range of game modes - Deathmatch, Capture the Flag, Clan Arena, more - to add variety.
Built-in player stats are on hand to track your progress. When you're confident of your skills, you might record games to show others later. And there are a host of other tools and features, ranging from a customisable heads-up display to a "spectator mode" and a smart anticheat system.