Command & Conquer: Red Alert 3/Cheats

Empire missions
In the final Empire campaign mission in Amsterdam, if you build Tanya and use her on the FutureTech building, the second stage of the mission gets bypassed.

In the final section of the Amsterdam mission, secret protocols are retained. If you have built the left secret protocol tree, you can get Sudden Assault and Final Squadron to bolster your forces and decimate theirs respectively.

Soviet missions
In the Mt. Fuji mission, using Natasha's snipe ability on the Emperor's Oni won't complete the mission until it's destroyed.

In the final Soviet campaign mission in New York, if you use a Star-rank Natasha from the top right corner of the map, she can airstrike the statue of liberty without dealing with the allied blockade at all.