Command And Conquer: Red Alert 2

Secret infantry units:
If you send spies into your enemy's Barracks, Battle Lab and War Factory you can unlock secret infantry units and will be able to build tanks and other units already promoted with one stripe. For example, in battle with the Soviets the Yuri Prime, Chrono Ivan, Chrono Commando, Psi Commando can be unlocked.

Free soldiers:
Build a Cloning Vat, then capture a Hospital. Shoot your own soldier, but do not kill him. Send him into the Hospital. When he comes out, you will receive an extra soldier of the same type sent in, without having to pay for one. You can use this with any unit.