RULES!
1. Post the game & put a dash & put the cheat/glitch/trick after it & put the version of the code (If it is different in different regions) in parenthesis
2. Put its description/how-to-do-it-thingy-thing-thing
3. Post its source from where you got it from
I'll start first by posting the level select & debug mode cheat codes for Sonic 1:
Sonic 1-Level Select (NTSC Version)
On the title screen, press Up, Down, Left & Right & if done correctly, you'll hear a ring chime. Hold A & press Start on the title screen to see the level select screen. While pausing the game, the corresponding buttons will do the following:
A-Restart the game (It'll lead you to the title screen again)
B-Slow-motion mode
C-Frame-by-frame advance
Start-Un-pause the game
Source: http://sonicology.fateback.com/
Sonic 1-Debug Mode (NTSC Version)
On the title screen, press Up + C, Down + C, Left + C & Right + C & if done correctly, you'll hear a ring chime. Hold A & Start on the title screen to start the game (Or hold A & choose whatever level you want on the level select screen.) to use Debug Mode. Wile pausing the game, the corresponding buttons will do the follwoing:
A-Jump/(While being Sonic)Cycle forwards through items(While being in Debug Mode)
B-Turn into an item
C-Jump/ (While being Sonic)Place item(While being in Debug Mode)
A + C-(While being in Debug Mode)Cycle backwards through items
Source: http://sonicology.fateback.com/