In between established titles and expansion packs, there have been 18 Command & Conquer games released over the last 15 years. Unless you're a real-time strategy fanatic, it's doubtful that you've been able to experience the entire breadth of gameplay the series has served up, and knowing where to start can be daunting for newcomers. Thankfully, EA's provided a pretty good jumping-off point by putting up four of the most notable entries in the series up on its Command & Conquer site as a free download.
For the low, low price of zero, you can pick up the game that started it all, Command & Conquer/; Tiberian Dawn, its hit follow-up Command & Conquer: Red Alert, and the one-two punch of Command & Conquer: Tiberian Sun and its expansion Firestorm. These four games represent a hearty chunk of the first five year's of Command & Conquer's history, which should prepare you well for the next chapter of the saga when it releases on March 16.
Source: Command & Conquer Classic [CommandAndConquer.com]