At the Main menu hold the 'Alt' key and type 'ufo cheat'. The message bar will confirm that the Cheat mode has been activated enabling you to enter the following codes to unlock the corresponding effect.

Gain One More of ALL Equipment:
Alt + (Keypad Plus)

Gain One of Each Vehicle Type:
Alt + X

Unlock Apocalypse Mission:
Alt + T

Display Number of Aliens in Buildings:
Alt + N

Flip Dimensions:
Alt + 0, 1, 2, 3, 4, 5, 6 or 7

Force Base Mission:
Alt + B

Force Overspawn:
Alt + S

Force UFOs to Crash:
Alt + C

Gain $100,000:
Alt + M

Show People Tube Connections:
Alt + Z

Test Alien Dimension:
Alt + G

Toggle Auto-Save:
Alt + A

Toggle Dimension Map:
Alt + D

Toggle Allow ALL Manufacture:
Alt + Q

Toggle Allow ALL Research:
Alt + R

Toggle Build Base Facilities:
Alt + F

Toggle Instant Project Finish:
Alt + P

Toggle View ALL UFOpaedia:
Alt + V