Codes!

THE INFAMOUS MINUS LEVEL
NOTE: Before attempting this, don't get discouraged if you can't make it work. I've tried tons of times, but I finally got it to.
How To Get There: Stand on top of the pipe in 1-2 that takes you to the flagpole. Break the 2nd and third ceiling bricks from the right. Face left and jump up. Hit the back of your head on the left side of the brick next to the pipe. If you hit it just right, you'll be pulled through the wall. Quickly jump into the left pipe to World -1, where there is no escape. If you wait too long, you'll reach World 4 instead.
NOTE: If you successfully make it into World -1, you either have to lose all your lives in it, or reset the NES, also, once you've slid through the wall, go into the second pipe if you would like to go to world 5.

CONTINUE
You can continue your game as long as you don't turn off your NES. When you lose your last life and the game starts over, hold A and press Start on the title screen. You'll start in the beginning of the world you left off in (i.e. if you die in 4-3, you'll start in 4-1.)

GAME GENIE CODES
It's pretty rare if you still have your SMB1 cartridge in working condition. It's even rarer if you have a working NES Game Genie. But for those lucky few, I present these codes.
NOTE: For unofficial codes discovered by people go to my Game Genie Codes page.
AATOZA = Start players 1 & 2 with 1 life.
IATOZA = Start players 1 & 2 with 2 lives.
AATOZE = Start players 1 & 2 with 9 lives.
VATOLE = Start player 1 with 8 lives and player 2 with 3 lives.
SXIOPO = Infinite lives for both players.
APZLGK = Super jump from a standing start only.
TPZLTG = Super jump from running only.
GPZUAG = Super jump from turbo running only.
APZLGG = Mega-jump from a standing start only.
APZLTG = Mega-jump from running only.
GAZUAG = Mega-jump from turbo running only.
>>MOON GRAVITY<<
The moon gravity effect is more than Super- or Mega- jumps, but it's harder to get used to. Stay with it and you'll find that it gives the game an added twist! You can control the height of your moon jumps. To jump really high, tap button A really quickly. To do low jumps, hold down button A for about one second. You can even jump over the flags! Just don't keep running once you do, because you'll have to reset the game.
YAZULG = moon gravity from a standing start
YAZUIG = moon gravity from a running start
YAZUYG = moon gravity from turbo running only
>>STAY BIG!<<
Use this code to ALWAYS stay big! You can still die if you fall down holes, though.
OZTLLX + AATLGZ + SZLIVO = Stay big.
>>WORLD-WARPING<<
YSAOPE + YEOZA + PEAPYA = Start on world 2
YSAOPE + YEOZA + ZEAPYA = Start on world 3
YSAOPE + YEOZA + LEAPYA = Start on world 4
YSAOPE + YEOZA + GEAPYA = Start on world 5
YSAOPE + YEOZA + IEAPYA = Start on world 6
YSAOPE + YEOZA + TEAPYA = Start on world 7
YSAOPE + YEOZA + YEAPYA = Start on world 8

GAME GENIE CODES:
DFDF-FAAD Start FILE A game with 2 lives
DBDF-FAAD Start FILE A game with 10 lives
7FDF-FAAD Start FILE A game with 50 lives
17DF-FAAD Start FILE A game with 100 lives
C2C1-D4AA Infinite lives
6D84-DF03 Infinite time
C26B-0FBF 1-up worth nothing
D5DF-FADD Allows you to select any world for FILE A
CB81-0D02 + E281-0D62 + 3C81-0DA2 Jump lower
>>SWITCH OFF EFFECTS IF YOU GET STUCK<<
CB81-0D02 + EC81-0D62 + 3C81-0DA2 Super jump
CB81-0D02 + E681-0D62 + 3C81-0DA2 Mega-jump
D62F-6DAE Invincibility does not last as long
9D2F-6DAE Invincibility lasts longer