Classic RTS Command & Conquer: Red Alert 2 is the latest freebie offered by Electronic Arts' "On the House" Origin scheme. On the House, for the benefit of those at the back of the class, works like this: Selected games on Origin are made free for a limited time, and if you grab it within that window then it's yours to keep forever. Previous games have included Bejeweled 3, Battlefield 3, Plants vs. Zombies, Dead Space, and Wing Commander 3: Heart of the Tiger. You can probably skip that last one, though.
The Red Alert 2 deal is the same thing. It's the full version of the alt-history RTS showdown between the United States and the Soviet Union, and it includes the Yuri's Revenge expansion as well, all at no charge. It may not be the freshest gaming experience ever—it was originally released in 2000—but it was very well-received when it was new, and I'd be willing to bet it holds up pretty well today, especially at the price.
Speaking of free games, don't miss out on the horrifically good Amnesia: The Dark Descent, which is currently being given away (but only until tomorrow) on Steam.