Extreme-G

Full weapons:
Enter "Contest" mode and press R at the bike selection screen. Enter ARSENAL as a name. A sound will confirm correct code entry.

Unlimited nitros:
Enter "Contest" mode and press R at the bike selection screen. Enter NITROID as a name. A sound will confirm correct code entry.

Fish-eye lens:
Enter "Contest" mode and press R at the bike selection screen. Enter FISHEYE as a name. A sound will confirm correct code entry.

Boulder race:
Enter "Contest" mode and press R at the bike selection screen. Enter ROLLER as a name. A sound will confirm correct code entry.

Turbo mode:
Enter "Contest" mode and press R at the bike selection screen. Enter XTREME as a name. A sound will confirm correct code entry.

Transparent track:
Enter "Contest" mode and press R at the bike selection screen. Enter GHOSTLY as a name. A sound will confirm correct code entry.

Slippery track:
Enter "Contest" mode and press R at the bike selection screen. Enter BANANA as a name. A sound will confirm correct code entry.

Magnification mode:
Enter "Contest" mode and press R at the bike selection screen. Enter MAGNIFY as a name. A sound will confirm correct code entry.

No gravity:
Enter "Contest" mode and press R at the bike selection screen. Enter ANTIGRAV as a name. A sound will confirm correct code entry.

Invisible mode:
Enter "Contest" mode and press R at the bike selection screen. Enter STEALTH as a name. A sound will confirm correct code entry.

Disable MIP mapping and textures:
Enter "Contest" mode and press R at the bike selection screen. Enter UGLYMODE as a name. A sound will confirm correct code entry.

Wireframe mode:
Enter "Contest" mode and press R at the bike selection screen. Enter WIRED as a name. A sound will confirm correct code entry.

Quick race finish:
Enter RA50 as a name. A sound will confirm correct code entry. Begin a race and quit during game play. The race will complete with a finishing position the same as whatever place your bike was when the quit option was selected.

Extreme Team racers:
Enter XGTEAM as a name. A sound will confirm correct code entry. Return to the name entry screen and enter the first name of one of the game's developers, from the credits. Some choices are Andy, Ash, Craig, Darren, Giles, Greg, Joe, John, Jon, Justin, Lloyd, Sadie, Shawn, Simon, Steve, and Omri. The faces of the developers will now appear on the bikes.

Fergus mode:
Enter FERGUS as a name and begin shoot 'em up mode. Fergus' face will now appear on the drones.

Roach motorcycle:
Complete the Meltdown circuit at the intermediate difficulty level. Alternatively, enter 51GG95 as a password for the Roach and all tracks; or complete all the levels.
Information in this section was contributed by blademaster614.

Neon motorcyle:
Complete the Meltdown circuit at the expert difficulty level. Alternatively, enter 61GGB5 as a password for the Roach, Neon, and all tracks.

Bonus track:
Complete the Meltdown circuit at the extreme difficulty level with a bike other than the Roach or Neon and finish first on all tracks. A small bonus track will be available in single and multiplayer modes. Alternatively, enter 81GGD5 as a password for the Roach, Neon, and all tracks including the bonus track. Note: Enter 61GGD5 as a password in the PAL version of the game.

Motorcycle inspection:
Enter the motorcycle selection screen. Press C-Left or C-Right to speed up, slow down, and stop the slow rotation of the motorcycles. Press C-Up or C-Down to tip the motorcycles to view the tops and bottoms.
Information in this section was contributed by Elliot Bowers.

All motorcycles:
Get all 100 kills in the "Shoot-Em-Up" mode.
Information in this section was contributed by Miguel1245.

Hint: Quick start:
When you first start, use nitro in order to speed past the others. This is a good way to get an early lead. Note: This trick will not work if not done early enough on Desert Track 2. However, it must be done after you go through the red haze.
Information in this section was contributed by Crash62491.

Hint: More speed:
When going down a steep hill, press Analog-stick Up, keeping it on the track and creating more momentum.
Information in this section was contributed by Crash62491.

Hint: Using ramps:
On a ramp, press Down to go higher and Up to go lower.
Information in this section was contributed by Fearthis818.

Hint: Easy points:
Use the following trick to get over 100 points easily. Enable the "Full weapons" code, then select "Shoot'em Up". Go around the track and on your third lap, stop before you finish. Keep pressing C-Right. Every time you get the Heat-Seekers, fire them all. It will give you more points each time.
Information in this section was contributed by Mark Snedeker.

Hint: Unlimited kills in shoot out:
Enable the "Full weapons" code and select Khan. Get red missiles and fire -- but you have to stand still.
Information in this section was contributed by Philip Van Gasse.

Glitch: Blow off the covers of the blocks:
In multi player mode, choose the Battle Arena, then select the upper right track. When you have started the game, each player should get any kind of weapon (except Heat Seeking). Get to one of the blocks and fire everyone's weapons all at once at the block. The glitch may not happen the first time you try, but repeat that process as many times as needed. Eventually instead of being yellow and black, it will be all different colors and then transparent.
Information in this section was contributed by Evilpenguin13.