Its been around for ages but Red Alert 2 is worth playing.
The story is excellent as you take on the soviets.
Sure it looks dated but the gameplay is brilliant.
Build you base. Your defenses (Taser coils are ace) your army and destroy the enemy.
The ability to play single player battles outside of campaign mode is excellent and you can also play the computers AI on maps that you create.
RTS fan - A must and a £5 a bargain