To enable cheat mode, press ALT-2-ENTER during gameplay.
Now press F10, and any cheat key:

G - God mode
W - Jump level
J - High jump
N - No clipping
I - Free items
T - Sprite test
D - Record a demo
V - Add a VLB
M - Get memory usage info
E - Restart game
S - Put game in slow-mo
Y - Makes the game look wiered
C - Mega-stats

Press C+T+SPACE to recieve the pogo stick, all the key cards, and
a lot of ray gun charges.

While at the map level, press SHIFT and TAB to bypass a city.