You can use Action Replay to unlock ships, tracks and parts using the following codes. In addition, there is game content in F-Zero GX which you *cannot unlock* unless you use Action Replay. Specifically, there are several custom parts which you can use to make a host of new racers which are only available in the Japanese version of F-Zero - but now you can enjoy them too!
To active these codes follow this process:
Purchase Action Replay (duh) - see below if you don't have it yet to figure out which version you want
Power off the cube
Put your normal memory card in slot A (if you purchased an Action Replay card with 16x, you can just stick that in slot A)
Put the action replay memory card in B (unless you put it in A)
Put the Action replay mini-DVD in and boot the cube
Select "Action Replay Codes"
Select US (only tested with US)
Check to make sure F-Zero GX doesn't exist - and pick "ADD NEW GAME" at the top
Enter F-Zero GX as the name
Enter (m) for the first code and enter the 4 code strings listed below for it. (This one is required for any of them to work)
Add as many other codes as you want from the list below
Select all the codes you want active and press start - it should auto "save hardware" and ask you to change discs to launch the game.
Race any time attack mode and place in the top 10 OR beat your record on any story mode to force a game save.
From here out you won't need Action Replay anymore - your memory card has been unlocked
NOTE: these codes were taken from the codejunkies.com website and some really hot ones were discovered by Avis (Custom parts, AX cups, staff ghosts) - many thanks to Avis!! Also, note that there are no letter "O"s in the codes, only zeros (0).
What are the AX Event-exclusive parts which are unlocked?
Body Mad Bull (900 kg) Body= B Metal Shell (950 kg) Body= B Blood Raven (600 kg) Body= C Rage Knight (440 kg) Body= D Galaxy Falcon (680 kg) Body= E
Cockpit Crazy Buffalo (600 kg) Grip= A Maximum Star (680 kg) Grip= A Hyper Stream (540 kg) Grip= B Round Disk (380 kg) Grip= C Rush Cyclone (300 kg) Grip= D
Booster Hornet -FX (700kg) Boost= A Shuttle -M2 (520 kg) Boost= B Velocity -J (420 kg) Boost= C Crown -77 (260 kg) Boost= D Mars -EX (270 kg) Boost= D
NOTE: All of the above codes provide access to new ships, parts or tracks, but do not provide an unfair advantage such as "infinite boost" or "single attack kill" settings. Those are available at the codejunkies.com website if you want them.