I found an entire huge depository of codes I am going to leave in text format since it is so huge. I did not do this, KarnAgeddon (woodchukx@hotmail.com) did. Thank him, and thank you. The Link

The codes in White were done by Mikrowerk. The codes in Blue were done by Synaesthia. The codes in light red were done by ANDY4U2CALL@webtv.net. Thanks.

OSVXUN: Code of codes; it builds up your HP points (the red energy meter dots) SUPER fast! I got 4 in the first area alone! If only there was a code to fill in the dots quickly...
VYOEPZ: 3-legged Rygar
GKPOTV: VCR-ish effect. Nothing useful, just interesting.

These are some visual codes

LPPEZS - Enemies are invisible (not too terribly interesting)
LPZEZS - Rygar is invisible
TAPESX - Night-time background (alter the first letter to find other background screen colors)
VAAENZ - New monster and background colors
GAAENZ - Other new monster and background colors
OXAENZ - Gold Rygar

These are some audio codes

ZZYTLP - Cool little sound clip before the game (guess who it is)
NTZTLO - Weird music
STYTLE - Weird music II
IYYTLP - Strange sort of evil sounding Rygar music
IZYTLP - Weird music III
ZZXTLP - Comical music
ZZOTLP - New music
LGXTLP - Rygar on flute

These are some monster codes

PXAEIZ + OXAELZ - Turtle monsters flee from you
OXAEIZ - High speed turtles
OXAEZZ + OXAEGZ - Catapulting turtles
LSUXUN - Rather tough monsters
LSXXUN - Rygar's death touch (all monsters are killed in one hit, including bosses)
YXAPSX - Ghost town (Almost no monsters at all)

These are variations on a Rygar code up there

GSVXUN - Super increasing tone
OKVXUN - Super increasing last
LSVXUN - The ultimate Rygar (both increase greatly. You gain a new energy capsule with every hit)

These are "hidden" levels and warps and things. On a few of the levels, you will travel along just to arrive at a pit or dead end. In those levels, explore leftward or die by jumping into a pit, and when you restart you will often be in a different place then where you were previously at.

GZTEZS - Begin in the Gran mountains, but altered a little bit
LPTEZS - Begin at beginning, but Gran mountains are changed
GPTEZS - Jump as soon as you possibly can if you want to live in this level
GPTAZS - Begin in a weird spot
YXAPNX - Eggdroppers in the first screen
SZYTLO - Begin way ahead in the castle with the annoying 4 note song.

YAZOIT: Mega-jumping Rygar.
APOEPO: super slow-motion.
TVSOAP- Hyper Rygar (settle down)!
SKUXIK- Infinite attack & assail (first go left & then jump back to the right).
POKAPO- Press start twice to change scenery.
GOKAGO- Press start twice for better colors.

ZOZOZO- Neat Auto-Die Feature!
KEEPEY- Fun Red Lines!
KILLEN- No enemies in first area.