Free modern online FPS games

The first list contains free FPS games that are set in modern day or in the past (WW2 for example). The gameplay in these online FPS games is usually similar to Call of Duty or Counter-Strike. Most of them have small urban maps and modern weapons. There are many free MMOFPS's where better weapons can only be purchased with real world money. All shooters have minimum system requirements listed (if available), with links to official website or download page.

Free Sci-Fi online FPS games

This second list includes free online first-person shooters that are unrealistic and usually set in the future. These FPS's often have similarities to shooters like Quake or Unreal Tournament. Most of them have relatively fast-paced gameplay and fictional weaponry like gravity guns and plasma weapons. There are around 10 games in this list that also have Linux and/or Mac versions. Some shooters may have very low player base or no players at all.

Other free shooters

The third list contains all free shooters that aren't online FPS games. Such as single-player FPS's with no multiplayer, third-person shooters and 2D shooters. Some are just short tech demos that show capabilities of a game engine or piece of hardware. There are lots of single-player indie FPS games, also couple single-player shooters with storyline, which were made free years after release.

Multiplayer FPS demos

The last list is of online FPS demos that still have servers with people playing. All of these demos are worth trying because they are different from regular free online FPS games. Almost all demos listed here have vehicles in-game, which is not very common in regular free-to-play shooters. The downside is that multiplayer demos usually have only one map and couple game modes, so it might get boring after a while.