I don't know how long this will last but at the moment there are a few Command and Conquer games that can be downloaded for free off of the EA site.
You can find it here : Command & Conquer Classic | Command & Conquer
The games that you can download are:
Tiberian Sun + Firestorm Expansion
Tiberian Dawn
and the Original Red Alert
Update: Seems like the beta is over o_O
As for the CnC 4 beta you go here and follow the instructions:
Get in the C&C 4 PUBLIC Beta! | Command & Conquer
Hope everyone that likes CnC takes advantage of this as I will.