Easily one of my favourite game packs made with OpenRA is Command & Conquer - Combined Arms, bundling together the Allies, Soviets, Nod, GDI and the Scrin. A new release is out in v0.91 expanding the content a fair bit.
Free to download and the main project is open source, using the freeware assets from the free releases of earlier Westwood RTS games that EA put up officially years ago. You've got nothing to lose but time here as it will suck you in if you love your classic real time strategy action. You can play it in single-player in the campaign, skirmish battles and online against others too.
A summary of the main points changed in this release:
- Chapters 4 and 5 of single-player campaign.
- Many improvements to existing missions.
- Added co-op variants of Duality and Containment.
- Increased variety of unit types AI uses to attack with in campaign.
- Restored old colour picker to allow darker colours.
- New music track, Recon Remix.
- New upgrades system to allow easier upgrading of units (V hotkey). Existing units superseded by an upgrade can be retrofitted via Service Depot with a new upgrade command.
- Added "Multi Queue Scaled" lobby option which makes production structures after the first in each queue cost significantly more until T2/T3 upgrades are purchased to make them cheaper.
- New "Composition Tournament" minigame map.
- New Team Fortress inspired minigame map.
- Added RAGL Season 14 maps.