X-Com: UFO Defense Cheats

Press [Ctrl] + C or [Ctrl] + G while in the battlescape screen to complete the current level with the results of the previous level.

When you target a terror site or crashed UFO with a ship, it does not disappear, giving you to get to it. If a terror site is in the darkness, send Interceptors one after the other to it. Hopefully, enough time will have passed to get the terror or crash site in the daylight.

When you finish building a new ship, load it with the weapons (Plasma Beam for both weapons is recommended). When it is fully charged and ready for flight, transfer it to another base. When it gets to the base, immediately send it out on patrol. Click the ship and you will see that its fuel is 0%. That means it will not run out of fuel. Make sure that when you send the ship out on patrol, send it to the opposite side of the planet or a long way from your base. This allows you to catch the ship after it attacks the enemy before it can return to base. This is best done with the FireStorm craft. They are cheap and quick to build. The firepower is just as good as the Avenger, which cost lots more and takes longer to build.
Information in this section was contributed by TheRaven.

Start a new game and build a base. Save it, and exit the game. Open the game folder and you should see folders named "GAME_1" to "GAME_10". These are the saved game folders. Enter the saved game folder that corresponds to the game that you saved. The order of the saves is the same as in the game ("GAME_1" is at top, etc.) After you open the folder with your save, make a back up of the "LIGLOB.DAT" file. Then, open the original "LIGLOB.DAT" file with a text editor. You will see some encrypted data. Press [Enter] so that text jumps to line two and line one is clean. Then, enter FF FF 7f in the line one over the encrypted data and save the file. Then, start the game, load your saved game, and enter your base. You should now have $1,713,399,398.