TSW .\\v4.0 - Because we're l33t like that.

Bonus Levels:
At the end of any Act, excluding boss levels, if you have at least 50 rings, there will be a huge ring you can jump inside to go to a bonus level.

Contributed By: RetroFreak, Source: Self

Config Mode
There is a code called control mode, which is required before activating this code.

To activate control mode, press UP, C, DOWN, C, LEFT, C, RIGHT, C at the title screen, but before pressing start to begin the game, press and hold A as you hit start. Now, rather than just being in control mode, you can enable config mode by pressing button B.

Sonic will morph into a ring, and the arrows can move him anywhere, even in the air, or through obstacles such as walls, floors, or even ceilings {GREAT FOR TELEPORTING!}.

You can change the item Sonic appears as by hitting A while in config mode. Button B makes Sonic normal again, and button C will place the sprite that you have selected for Sonic to appear as. For example, you press B, and Sonic becomes a ring, press C to make a ring appear exactly where the ring icon is.

WARNING!: This will distort several different things, such as the score, time, and other various icons throughout the game such as the finish sign at the end of the first two acts of each zone that spins when Sonic shoots past them, and the small score icons that appear whenever Sonic jumps high enough after finishing a level. That is all I have noticed in my time playing the game, but there could be more out there yet. The game is still quite playable this way, so it should not hurt you.

Contributed By: Ryo Death, Source: Self, when I tried combining level select and control mode 10 times

Control Mode
On the title screen, push Up, C, Down, C, Left, C, Right. If you don't hear a chime, press C four times, then press Up, Down, Left, and Right until you do hear a chime that confirms the code.

This code gives you extra techniques. While the game is paused, press A to reset the game, press B to play in slow-motion, or press C to make the game advance one frame at a time.

Contributed By: RetroFreak, Source: world-of-video-games.com

Level Select
At the title screen, press Up, Down, Left, Right. You should hear a noise like a ring being collected. Then, hold A and press Start for a level select!

Contributed By: MrJeff, Source: Unknown