On the race/car selection screen, type:
KEVWOZEAR - This unlocks all races/all cars/all car upgrades
IWANTTOFIDDLE - Encrypts/Decrypts all text files

Network Game:

Type in 'JOYRIDER' at the network race-type selection screenand you'll get access to all the cars for network play. You'll need to select 'all' cars in the options screen to use them. This works with manual or random car selection.

To get the powerup marked in Purple, type in the word in green while playing.
Type slowly, allowing the game to perform whatever function is bound to each key before pressing the next key.
Instant Handbrake: SPAMFORREST
Oh Dear, Bodywork Trashed: BIGBOTTOM
Underwater Ability: GOOGLEPLEX
Mega Bonus (loadsa cash): GIVEMELARD
Pedestrians Glued to Ground: SPAMSPAMSPAMSPAM
Giant Pedestrians: SMALLUDDERS
Explosive Pedestrians: SUPERHOOPS
Turbo Pedestrians: FUNNYJAM
Gravity has gone Strange: IHAVESOMESPAM
Bouncey, Bouncey!: CHICKENFODDER
Oh Dear, Jelly Suspension: BUYOURNEXTGAME
Pedestrians Shown on Map: ILOVENOBBY
Pedestrian Electro-B**tard Ray: RUSSFORMARIO
Blind Pedestrians: HAMSTERSEX
Pedestrian Respawn: NAUGHTYTORTY
Pedestrian Harvest: TRAMSARESUPER
Vesuvian Corpses: ISLANDRULES
Gravity from Jupiter: RABBITDREAMER
Free Repairs (not official): SPAMFRITTERS

Full game cheat and ingame editing:

Type 'IBETYOUCANTPRINTCUNT' when in a race.

F4 - Swap between ingame modes, mostly for car and track editing.

Edit Mode - Cheat

0 Bonus
1 Mega bonus
2 Pedestrians glued to ground.
3 Giant pedestrians.
4 Explosive pedestrians.
5 Hot rod.
6 Turbo pedestrians.
7 Invulnerability.
8 Free repairs.
9 Instant repairs.
SHIFT-0 Timer frozen.
SHIFT-1 Underwater ability.
SHIFT-2 Time bonus.
SHIFT-3 Oh dear, bodywork trashed
SHIFT-4 Blow yourself up
SHIFT-5 Frozen opponents
SHIFT-6 Frozen cops
SHIFT-7 Turbo opponents
SHIFT-8 Turbo cops
SHIFT-9 Lunar Gravity
ALT-0 Pinball mode
ALT-1 Wall climber
ALT-2 Bouncey-Bouncey
ALT-3 Oh dear, jelly suspension!
ALT-4 Pedestrians shown on map.
ALT-5 Pedestrian electro-bastard ray. EXCELLENT!
ALT-6 Greased tyres
ALT-7 Acme damage magnifier
SHIFT-ALT-2 Instant handbrake
SHIFT-ALT-4 Turbo!
SHIFT-ALT-5 Mega-Turbo!!
SHIFT-ALT-6 Blind pedestrians (to be used in conjunction with horn for a laugh)
SHIFT-ALT-7 Pedestrian respawn
SHIFT-ALT-8 5 free recovery vouchers.
SHIFT-ALT-9 Solid granite car!
CTRL-0 Rock springs!
CTRL-1 Drugs!
CTRL-2 Grip-o-matic tyres
CTRL-3 Pedestrian Harvest EXCELLENT!