Penny Racers
Quick Select
When the computer is choosing what parts to take, press the A button rapidly. This will make the computer select faster. The good part about this code is that if the computer is choosing from you, you have a better chance of them taking nothing.
Credit Goes To:Ryan and Dan Charnow via IGN 64
 Quick Select
 Bonus Mode
 Trade Parts
 Turbo Boost
 Better Weapons,
 Extra Points,
 Class Advancement
Bonus Mode
To access the bonus modes you must meet the following criteria:
Mode B: Get a Bronze Medal or better on every track of mode C.
Mode A: Get a Bronze Medal or better on every track of mode B.
Mode AA: Get a Bronze Medal or better on every track of mode A.
Credit Goes To:Supersorce via IGN 64

Trade Parts
Ever wonder how you could get a CPU winner to trade you something for something when he takes a part away from you? Well, go to the title page and press start when you turn on your system and select "OPTIONS". Move your control stick until "PARTS OPTIONS" is highlighted and press the A button twice. Leave "STEAL PARTS" on and turn "TRADE PARTS/SWAP PARTS" on. Now, the CPUs will steal one of your parts (and sometimes none) and no matter what will give you something of his.
Credit Goes To:Kevin Williams via IGN 64

Turbo Boost
Right before starting the race, press the gas button when the last light is about to turn green. If you are playing one player mode and seem to have a hard time starting off, rev the engine until it gets close to 5. Once it starts you off, you should get a faster start.
Credit Goes To:Ryan Charnow via IGN 64

Better Weapons,Extra Points,Class Advancement
Win a medal on each level in class C,B, or A and you get better wepons, extra points, and class advancement in B, A, or AA.
Credit goes to: D.M.C. via N64cc