About five years ago or so, id software released a game that was originally to be an expansion to Return to Castle Wolfenstein. For some reason, instead they released Wolfenstein: Enemy Territory as a free download in 2003. It was online multiplayer only, and it became a bit hit.
At the time, I was active in the online forums at the UserFriendly webcomic. Over time, a couple members hosted dedicated servers for the UserFriendly community. The members were fairly decent at maintaining "house rules" — basically, things like no cheating/exploits, no spawn killing, etc. and so forth. For the most part, people respected the rules, and just got together for the fun of the game. I logged many, many hours in W:ET in those days.
W:ET is a team-based, class-based, objective-based first-person shooter that pits the Axis (Germans) against the Allies (Americans). In the majority of scenarios, the Allies are on the offense and have to accomplish some objectives, while the Axis are on the defense and have to prevent the Allies from accomplishing their objectives. (There are maps that switch the offense and defense, but they are the exception, at least in the standard set.)
Each class has certain, special abilities. Engineers can build and repair structures and vehicles, plant and disarm dynamite, and plant landmines. Covert Ops can steal enemy uniforms, which allows them access to doors only accessible to the enemy team. They can also plant C4 charges, which can quickly blow up small structures (gun emplacements, for example). Field Ops can call in air support, spot land mines, and resupply teammates with ammo. Medics can heal and revive teammates. Soldiers have access to heavy weaponry.
The game is very focused on team play. While it is possible for a single Engineer to play the game and accomplish all the objectives solo, he is far more effective if there's a Medic nearby to heal or revive him when he goes down, and a Covert Ops to let him in the back door. A Soldier with a portable gun emplacement makes a fair defense, but he will run out of bullets or die if he gets hit enough. Position a Medic and a Field Ops nearby to keep him healed and full of ammo, and he can sit there mowing the enemy down all day.
Probably my biggest complaint in the game is how heavily it focuses on the Engineer. Just about every objective requires an Engineer, whether you're building or destroying barricades, planting or defusing dynamite at critical locations. The other classes are very effective at what they do, but in the end, they come down to how well they support the Engineers at doing their job. Now, not to toot my own horn, but I got very good at being an Engineer (I tend to do better at objective-based games where the objective is something other than "kill the other team"), so although I tried to spend time building up my skills in other classes, I found I had to keep switching back to Engineer to get the job done. And, as I mentioned before, when other people who excelled at their "support classes" were doing their job well, the results were devastating.
I have many fond memories of that game, including my "Al Bundy" moment were I was on Axis defending our Very Big Gun when a team of Engineers slipped in and planted four packs of dynamite on the controls. I ran in, wiped them out, and while they were busy congratulating each other in the chat window waiting in the respawn for the kaboom, I defused all four dynamite packs just in time.
The game is still available through official channels, on the Enemy Territory website in the "Downloads" section; and although I'm all but forgotten in the UserFriendly crowd these days, I have noticed they still have the daily "W:ET Bar" post on the forum.