Activate the following cheats by entering the corresponding codes at "Extras" > "Enter Code".

CHEAT FUNCTION - CHEAT CODE
Mob car - "BIGTRUCK"
Pimpster - "RUTTO"
Rocket car - "KALJAKOPPA"

Unlock the following cars by performing the corresponding tasks:

CAR - HOW TO UNLOCK
Bonecracker (Derby) - Earn a Gold medal in the Bonecracker Demolition Derby.
Bullet, Lentus and Ventura (Race) - Beat the Race Class Stage 2 with a Bronze or better medal.
Canyon (Street) - Earn a Gold medal in the Canyon Beat The Bomb Race.
Crusader (Street) - Earn a Gold medal in the Crusader Deathmatch Derby.
Flatmobile (Special) - Earn a Gold medal in the Flatmobile Beat The Bomb Race.
Grinder (Derby) - Earn a Gold medal in the Grinder Demolition Derby.
Insetta (Race) - Earn a Gold medal in the Insetta Carnage Race.
Lancea, Daytana and Fortune (Race) - Beat the Race Class Stage 1 with a Bronze or better medal.
Mob Car (Special) - Earn a Gold medal in the Mob Car Carnage Race.
Road King (Street) - Earn a Gold medal in the Road King Beat the Bomb Race.
Shaker, Blaster XL and Banger (Derby) - Beat the Derby Class Stage 1 with a Bronze or better medal.
Sparrowhawk, CTR Sport and Vexter XS (Street) - Beat the Street Class Stage 1 with a Bronze or better medal.
Speedshifter, Terrator, Speeddevil, Bullet GT and Sunray (Street) - Beat the Street Class Stage 2 with a Bronze or better medal.
Splitter, Switchblade and Venom (Derby) - Beat the Derby Class Stage 2 with a Bronze or better medal.
Truck (Special) - Earn a Gold medal in the Truck Deathmatch Derby.

For quick money, forget about winning the races. Instead, when first starting off, turn around and drive backwards to as many crashes as you can. You get better payout for crashes than for winning the race.

ACHIEVEMENT - HOW TO UNLOCK
Go That Extra Mile (40 points) - Drive 1001 miles.
Insane Air (20 points) - Jump 10m high with your car on a Carnage or normal Race.
Burnin' Up (20 points) - Burn nitro continuously for at least 10 seconds in a normal race.
Schoolmaster (15 points) - Be the top wrecker in 50 races.
Keep on Truckin' (15 points) - Win 15 Derbies.
Lightning Fast (15 points) - Get the fastest lap of the race in 50 races.
Windshield Breaker (15 points) - Get 30 Crash-Out crash bonuses.
That's a Start (10 points) - Complete your first cup in FlatOut Mode.
On a Roll (20 points) - Complete your first class in the FlatOut Mode.
Give Me Glory (20 points) - Get 1 million points in the Carnage Mode.
Carnage Master (30 points) - Complete Carnage Mode.
Carnage Wizard (40 points) - Complete Carnage Mode with all gold.
Race Master (30 points) - Complete FlatOut Mode.
Race Wizard (50 points) - Complete FlatOut Mode with all gold in all cups and events.
Fragman (20 points) - Complete all the Deathmatch Derbies in Carnage Mode with all gold.
Elite Stuntman (20 points) - Complete all the Stunts in Carnage Mode with all gold.
Master Stuntman (15 points) - Complete all the Stunts in Carnage Mode with silver or better.
Bomb Car Racer (20 points) - Complete all Beat the Bomb's in Carnage Mode with all gold.
Derby Expert (20 points) - Complete all derbies in FlatOut Mode with all gold.
Pro Racer (20 points) - Complete all Time Trials in FlatOut Mode with all gold.
Holy Slammaster (15 points) - Get 100 Slam crash bonuses.
Master Trucker (10 points) - Win a Head On or normal race with the Truck.
Crashman Returns (30 points) - Win all Bonus Race Single Events with the Flatmobile.
Real Habanero (60 points) - Win Derby class in Flatout Mode (all cups gold) with an unupgraded Chili.
Car Rental Service (40 points) - Own at least 6 vehicles at the same time.
Used Car Salesman (35 points) - Sell 10 cars.
Pro Tuner (25 points) - Own a fully upgraded Street class car in FlatOut Mode.
Total Domination (30 points) - Achieve all 4 "Top Driver" awards in one race and win the race.
Bang'n'Slam (10 points) - Get 5 Slams in one race.
Windshield Tester (15 points) - Get 10 Crash-Out bonuses
Hostmaster (10 points) - Host 10 online matches.
Online Race Veteran (10 points) - Win 10 online Races.
Online Race Pro (20 points) - Win 25 ranked online Races.
Online Stunt Veteran (10 points) - Win 10 online ragdoll Stunts.
Online Stunt Pro (20 points) - Win 30 ranked online ragdoll Stunts.
Online Derby Veteran (10 points) - Win 10 online Derbies.
Online Derby Pro (20 points) - Win 20 ranked online Derbies.
Live Ammunition (30 points) - Wreck 10 opponents in online races.
Live to Crash (25 points) - Wreck 3 or more opponents in the same online race.
Live to Race Clean (25 points) - Complete an online Race without crashing into anyone, and without ragdolling at all.
Fragmaster Live (20 points) - Get a frag streak of 5 or more in an online Deathmatch Derby.
Perseverance (10 points) - Don't give up after losing a certain number of times.
First Blood (15 points) - Wreck an opponent's car in Carnage or normal race.