We do not provide any information in regard to, or condone, the use of any online cheats on Red Alert 2 - but there are bugs which can be used as such, dependent on your version of the game.
Black Eagle/Harrier Repair
Planes automatically heal whilst on an AFC - albeit very slowly. You can actually get them to heal far more quickly by force-firing on a service depot or naval yard. It will often take several attempts, but eventually the plane will land there and be repaired.
Unlimited Infantry (Will not work with Patch 5+)
This is a bad bug - it was sent in by one of our fans and has since become well known. Play as Soviets, build a cloning vat - now put walls all around your barracks - so units cannot come out of them. When this happens you will be refunded the money that you spent on the unit - yet if you have a cloning vat, a unit will still be produced there. So basically, you can produce as many units as you wish for no cost whatsoever.
School Bus Trick
For the trick to work you must have either Yuri or a PSI Commando. Force fire on a school bus and they will mind-control it. You can now place up to 5 infantry units in it - the most useful units to place inside are Crazy Ivan and/or engineers. Once inside you can now drive the bus - the enemy (if a computer) will simply ignore it - they cannot see it basically. This means you can drive it right into the middle of their base (very useful for uncovering the shroud) and then simply unload your troops wherever you desire.
Hospital Bug (Will not work with Patch 4+)
Quite a major bug - capture a hospital with an engineer and build a cloning vat. Any units that you send to the hospital to be repaired will produce a clone when they exit the hospital! Meaning you can purchase infantry for nothing.
Desolator Bug (Will not work with Patch 4+)
After you have deployed the desolator force fire on something out of the radiation range - the range and power of the radiation will double! This makes the desolator practically unstoppable against ground units.
Build Anywhere (Will not work with Patch 4+)
Place a structure down - but don't release the mouse button; drag the building onto the sidebar, move the screen with the arrows on your keyboard. You can now place the building anywhere; dumping a grand cannon in the enemy base would be very evil!
Prevent Bridge Repair (Will not work with Patch 6+)
This cheat prevents the opposition from repairing bridges. First of all destroy a bridge, secondly send a submarine/squid/dolphin into the gap created by destroying the beige. Now even if the enemy sends engineers into the bridge repair huts they won't be able to repair it. Another method is to make a chrono legionnaire force-fire on a bridge repair hut - he will erase it permanently.
Invisible MCV (Will not work with Patch 4+)
Build a terror drone and then build a mcv (you'll need a supply depot). Place your MCV in an open area (don't deploy it yet), place the terror drone into it (force-fire) and then deploy it; after that you just click on your MCV and undeploy it and it disappears. You can build off the MCV, but the enemy cant see it or kill it - you have to wait until they quit before you win the game however.
Please note: We do not condone the use of any of the above cheats online - especially in tournament games. The use of bugs is classed as cheating on XWIS. If you use these cheats then players may well report you to XWIS who could serial ban your game.