OpenRA is a free and open-source real-time-strategy (RTS) clone of Command & Conquer - Red Alert by Westwood Studios. The game offers similar look, feel and gameplay as the original Red Alert, but with extra tweaks like: improved user interface, improvements to team balance, new multiplayer maps and much more.
Intel powered Mac
Mac OS X 10.5 or newer
Internet connection (for multiplayer)
Free download at openra.res0l.net