Spring Engine is a surprisingly capable open source 3D RTS game engine, which can help you develop your own games, or (if that sounds too much like hard work) just play those already created by others.
The system supports everything you'd expect from a modern game. It can handle a huge number of units, for instance (up to 5,000), spread across massive 3D maps. There's scope for very powerful enemy AI. A quality physics engine, deformable terrain and dynamic and reflective water means everything looks and behaves as you'd expect, and you're able to play on your own, on a LAN or online.
And a quick look at some of the games created by Spring Engine shows that individual teams have taken this technology in many different directions. A World War 2-based game offers huge tank battles, for instance, while another takes a Star Wars theme. Some games focus on building an economy, others are all about the action, and there's a whole group based around Atari's Total Annihilation.
The download we have here is just an engine. In order to actually play something you'll need a game and a map, and that's where life becomes a little more complicated. Some games can be added after the engine, but others already come with engine bundled, so providing everything you need in a single package.
What to do, then? If the idea of free RTS gaming sounds interesting then we'd recommend you start by taking a look at the list of games on the Spring Engine site. Follow the link to any that you think you might like, and you'll find information there on exactly what you need to download.
Version 98.0: "Overall performance should be improved in comparison to spring 96.0. We've switched to SDL2 which allows to switch between windowed and full-screen mode at runtime."