Pause Startup: this is more of a GBA trick than a Spyro code.
Press + at the Gameboy startup screen and (Nintendo) disappears. It just sits there with Gameboy on the screen untill you hit A,B or the directional pad.

Quick Start:
hold + + after you turn on the game.

Infinite Lives - This is the code from speedway.spyrothedragon.com where you can play the "Super Spyro Speedway" game, score 2000 points and get the code.
Enter this code at the 'Press Start' screen and you will always have 5 lives.

Turn on the Fairy Magic - Now you can't drown in any world with water
Enter this code at the 'Press Start' screen.

Headbutt ability - without defeating Crush you can break boulders by charging into them.
Enter this code at the 'Press Start' screen.

Unlock all breath abilities - you can select from all breath abilities and they never run out. You have Flame, Ice, Super Flame, Super Ice and Electric.
Enter this code at the 'Press Start' screen.

Atlas warp ability - without defeating Ripto you can warp to any world with a page in your atlas.
Enter this code at the 'Press Start' screen.

All worlds visible in atlas - You can view the page for every world, even those you have not been to yet.
Enter this code at the 'Press Start' screen.

Open all portals - Walk right into any world you please.
Enter this code at the 'Press Start' screen.

Turn Spyro Blue - This code is available on the "Big Fat Liar" DVD.
Enter this code at the 'Press Start' screen.

Don't wanna be blue - Save your game, exit and turn off the GBA. When you come back he should be normal.

Agent 9 - Infinite Ammo - The rapid fire ammo will also be infinite when you pick it up
Enter this code at the 'Press Start' screen.

Agent 9 - Infinite Shield - Agent nine can't be hurt but he can be knock around and will die if he falls into the thorns.
Enter this code at the 'Press Start' screen.

Unlock Dragon Draughts - Play the Dragon Draughts mini game without defeating Ripto.
Enter this code at the 'Press Start' screen.

Glitch: Lots of lives - Go to an area where you can fall to your death (with or without water). Jump in until you are down to one life. Then, jump off and while you're falling, press Start and select "Exit Level". When you return to the Home area your lives will say "0". Next time you die it will say "05". I've tried dying several times. It goes down to "00" and then "09". So I have no idea how many lives you get. Edited and Expanded from info provided by Jeremy Sandoval

Undo: use the infinite lives code, save your game, turn of the GBA, When you come back you should only have 5 lives.