FlightGear has scenery that contains environments from all around the world.
Still in developement
Uses the GNU GPLv2.
Getting stuck upside down
After a crash a pilot may be stuck in an upside down position with no way to recover.
One of the only free RTSes available being actively developed
When it comes to a completely free RTS that is being actively developed there are few options, or really only one, this game. So for that part, the best option available as it is really the only one. While there may be some free to play RTS options available they do rely on in game transactions where this title will have none.
Free and open-source
Even if the current developer team chooses to no longer work on the game, others can keep contributing to the source code and modify it.
Addictive Aoe style game that works on nearly any OS
This game is very similar to Age of Empires, so it should be right up the alley for fans of AoE as they will be familiar with the style of gameplay.
The game will run on Linux, OSX and Windows, which is great for people who dual boot or just use multiple OS's across their devices. No matter the OS used, one can install and play this game.
By being in active development the game allows for frequent bug fixes as well as frequent new content. While it is still in alpha, the state of the game playability is pretty good with new features being added all the time.
Straightforward RTS gameplay
0 A.D. brings familiar straightforward real time strategy gameplay for free using open source code. With a mini-map in the bottom corner, resources shown in a bar at the bottom and of course unit selection through double clicking, all of the major elements found in most RTS games is here, making for a very familiar game for those that are used to the genre.
Any considerable amount of units causes important slowdowns due to problems with the unit pathfinder.
Currently in alpha
0 A.D. is an ongoing open source project that is as of yet feature complete.
4) America's Army
The Official Game of the U.S. Army.
Works well on most hardware
This is not very a resource intensive game, meaning it should work just fine on lower end systems.
Easy to try
The game is free, meaning there is no cost to test the game in order to see how it performs or if the gameplay is appealing.
Very good graphics
The game has pleasant and realistic graphics that should appeal to anyone looking for a real life army shooter.
American Army Video Adds
Since this is a game designed as propaganda for the American army; it features videos promoting the American army. Although they can be skipped, they increases the game size, and disrupt the experience.