An Unreal Tournament modification formerly known as Tactical Ops, that was first released 12/22/1999. It is a round-based first person shooter semi-realistic team-based action game very similar to Counter-Strike, but the Unreal Tournament engine renders larger, more detailed maps than the Half-Life engine.
While also providing a single player mode against bots (computer controlled players) it clearly is an online game.
Gameplay
First you have to choose which of the teams (Terrorists or Special Forces) to join. At the beginning of each round you have to buy the weapons and equipment (armor, night goggles) you need, but in the first round you usually only have money to buy some armor or a slightly better weapon. Together with your team members you then run around the level and try to achieve your teams mission goal (eg. placing a bomb) while preventing the other team from doing the same (eg. defusing the bomb). Your team can also always win by completely killing everyone from the opposite team. Other examples of mission goals would be "Rescue the Hostages", "Escape the map through a special door", "Access a pad in your teams base while defending your own pad" and so on. All this has to be done within the Round Time Limit to prevent the teams from camping endlessly.
You earn money everytime you kill an an enemy (and lose the double amount for killing a teammate), pick up special items placed randomly on the map and manage to stay alive till the round ends, when your team wins a round and when you sell your weapon before buying a new one.
Semi-Realistic?
If you get hit, there is no way to get healed again. You won't bleed to death and you'll still be able to run, but your hit points are precious. If you die, you stay dead until the next round begins and of course drop your equipment for others to pick up. Together with the realistic (read: high) weapon damage this leads to a very strategy-based gameplay, and this is what makes this game so interesting.
The weapons are based on real world rifles, guns, pistols and grenades, but their names had to be changed (eg. Black Hawk instead of Desert Eagle) since from the moment the game went commercial, real weapon names were a trademark violation.
How to get it?
You can buy the retail version in the US and Europe or just download a free version of the game without some of the maps if you already have Unreal Tournament.
Official web page: http://www.tactical-ops.to
Update: The (attow) last patch of TO, version 3.4, now is a retail only patch. That means you have to buy the game, or stick with 3.3.2. New additions are minor things like the fancy binoculars, a slightly better night vision and somewhat improved weapon balance. And, of course, the usual bugfixes (and regressions).