OpenRA is a free and open source real time strategy game engine supporting early Westwood games like Command & Conquer (C&C) and Command & Conquer: Red Alert.
Almost an year back, I tried OpenRA but couldn't get it to work. Recently I gave it a shot again and was pleasantly surprised at the level of polish.
The game is absolute pleasure to play. The graphics may look a bit dated but they have their own charm. Gameplay is as addicting as it used to be and brings back old memories. Check out the video below:
OpenRA can be currently played as online multiplayer game only. However, you can create a single player game by selecting bots as enemy opponents with three different AIs (easy, medium, hard).
If you have original Red Alert files, you can use them with OpenRA. But, they are not necessary. Both original C&C: Red Alert and C&C has been recreated as mods and can be downloaded from game menu itself. They are not just clones of the original versions but provide improved and rebalanced gameplay.
Download OpenRA (Arch Linux, Debian, Ubuntu and Fedora Packages)