In an effort to help promote the forthcoming Command & Conquer 4, EA has made C&C: Tiberian Sun + Firestorm expansion free to download and play. At 1.2GB its a big download, but well worth the time to play another classic game in the series for zero dollars.
You can access all game downloads at the EA Command & Conquer Classic web page. Just scroll down to find the download button for each game. EA would also like you to click the big pre-order button for C&C 4 while you are there.
Command & Conquer: Tiberian Sun free to download