EA has pushed out Command & Conquer: Tiberian Sun as well as its expansion pack Firestorm absolutely free of charge. Released in '99, Tiberian Sun is a sequel to 1995's Command & Conquer: Tiberian Dawn and continues the struggle for world domination between the Global Defense Initiative and the Brotherhood of Nod. Less than a year later, Firestorm was introduced and it follows the events of the GDI campaign of Tiberian Sun.
It seems the company is trying to stir up some attention for its upcoming title, Command & Conquer 4: Tiberian Twilight. Slated for debut on March 16, Tiberian Twilight will conclude the Tiberium saga and is set in 2062, 15 years after the events of Tiberium Wars and 10 years after the events of Kane's Wrath. Tiberium is spreading across Earth at a rate which will render the planet uninhabitable by 2068. Kane is on a mission to prevent this, and the game will complete his story, revealing his identity and motives.
It's worth noting that both Command & Conquer: Tiberian Dawn and Command & Conquer: Red Alert are also available as freeware -- and have been for some time.