Building Glitch
Click the paradrop icon when you have a building available, but before you choose its site. Next, click anywhere on the map. The building will appear there, whether you've explored it or not.
Infantry Glitches
Capture a hospital after you build a cloning vat. Shoot one of your own infantry just a little bit, then heal him in the hospital. New infantry will come out of the cloning vat for free.
Build a barracks and a cloning vat, then build walls around the barracks. When you train an infantry unit in the barracks, he won't be able to come out, and you will be refunded. However, his clone will still come out of the cloning vat.
Unlockables
Crono Commando and PSI Commando can all be unlocked if you capture the other team's battle lab. To get Ivan and Yuri, you'll also need to build a Soviet Barracks.
Free Promotions
Put a spy into your enemy's barracks. Now, every soldier that comes out of your barracks will have one automatic promotion bar.