OpenRA is an open-source implementation of the Red Alert engine, which essentially allows you to play classic real-time strategy games along the lines of Command & Conquer and Command & Conquer: Red Alert. Not clones, as such, more what the authors call "reimaginings", similar but featuring improvements on the originals.
If you don't remember the first games, then they're classic RTS. In a typical skirmish game, you'll start with a single base, and must build refineries to begin collecting resources and earning money. Once your economy is up you're able to build defences, like turrets, guard towers and more. Then you can start generating soldiers, tanks and so on, which will probably be very useful as the other inhabitants of the map will have discovered you by now (and they're not friendly). But if your base is powerful enough then you'll be able to see off the threat, seek out your opponents home, and wipe it from the face of the earth.
The authors are also adding support for single player campaigns, which will provide more of a mix of missions. Although if you need extre entertainment right now, then you can always join a multiplayer game, or set one up yourself.
The problem? OpenRA's graphics are, well, a little basic. To put it politely. The default settings on our test PC left most units as such tiny groups of pixels that we could hardly tell what any of them were. (Although playing around with the settings and enabling "pixel doubling" did help with this, just a little.)
Forget the visuals, then, this is all about the gameplay. But if you like this kind of classic RTS game then OpenRA will prove an engrossing experience which includes more than enough features to keep you entertained for many, many hours.