Add the following command line parameters to the Midtown Madness shortcut:

-allrace - access all races
-allcars - access all cars

In single-player mode, press Ctrl+Alt+Shift+F7 and hold them down until a prompt appears. Then type one of the following codes:

/NODAMAGE - disables damage
/DAMAGE - turns damage back on
/BRIDGE - bridges will move more quickly
/PUCK - your car will have no friction
/GRAV - enables low gravity
/UFO - airplanes will turn into UFOs
/BIG - large people
/TINY - small people
/NOSMOKE - turns off smoking tire effect
/SMOKE - turns on smoking tire effect

Enter the following as your player name to unlock the respective cheat:

Showme Cops - police displayed on the map
Big Bus Party - all city bus traffic
Tiny Car - all compact car traffic
Jet Planes - all airplane traffic
vacompact - select the VW Bug in cruise mode to drive a VW Rabbit
vavan - select the Ford F350 in cruise mode to drive a van
valimo - select the Ford Mustang GT in cruise mode to drive a limo
vataxi - select the Cadillac in cruise mode to drive a yellow cab
vaboeing_small - select the City Bus in cruise mode to drive a mini-jet