OpenRA is a free real-time strategy game engine compatible with mods to classics from Westwood Studios like Command and Conquer and Command and Conquer: Red Alert, which in fact you can download through the OpenRA interface.
After installing OpenRA (which is not a game in itself but rather the platform that supports games) you can automatically download a remake of Command and Conquer: Red Alert. You'll immediately be able to start playing, either by yourself (against the computer) or online, where you'll find people to play against who match up to you in terms of skill.
The main novelty offered by OpenRA in terms of the Command and Conquer classics are the graphics, which have been adjusted for more modern screen resolutions. In fact, OpenRA lets you enjoy the same real-time strategy from the nineties with brilliant new graphics that hold up to today's standards.
In any case, what makes OpenRA so special is that it's very easy to apply different mods. The official modding forum has more than 100 threads where users share the things they've created.
Nowadays, OpenRA might be the best way to enjoy the Command and Conquer classics. A real treat for lovers of real-time strategy with that old-school touch.
