How has nobody recommended Supreme Commander (Forged Alliance)? It's the inspiration for other games in this thread like Planetary Annihilation and Beyond All Reason. It's so good that when the official servers shut down the community banded together and formed their own server and dev team to continue regular balancing and updates. It's on steam and works on Linux with minimal tinkering. You can either play the campaign on the Steam version or head straight over to Forged Alliance Forever, the community-hosted server, to access the campaign (with added multiplayer support!), all the new patches and mods, and play some multiplayer games too.