Spring (formerly TA:Spring) is a full 3D RTS. The game is played by getting one or more mods for the engine (which is completely GPL). The original goal was to run all Total Annihilation mods and 3rd party stuff. It seems to have passed that long ago and is now a damn complete game.
The best part of the game is the ability to control specific units from an FPS perspective. Selecting the unit and pressing ‘C’ puts you into FPS mode and you can control that unit directly. This is really cool when you try it out on aircraft in fierce dogfights and adds a really clever element to the gameplay, especially since full 3d air combat means you can fly underneath and above other aircraft (unlike most other RTS games) . There can be thousands of units in the game at the same time, but micromanaging isn’t a problem because some units have a rudimentary AI inbuilt which you can set. The terrain is great and deforms with sufficient damage causing craters and the like.
There are units in every medium and with the BA and other similar mods you’ll have units on sea, air, and land. The mods are usually well constructed and the sides balanced, so the multiplayer aspect of Spring (which is the most important part) is rarely neglected.
Getting the AIs to work seems to be non-trivial, so single player missions may not be extremely easy to setup, but the Random Enemies scripts should keep you busy while you learn. The Setup Guide is well written and easy to follow and they even have a Debian package repository to make installation of the binaries simple. There’s also a section for Ubuntu in the Setup Guide, so things can’t be much simpler.
