Sonic 3D Blast

Screen capture:
Press [F9] during game play to save an image of the current screen in the game directory.

Donkey Kong appearance:
Go to some of the trees on the first level to see what appears to be Donkey Kong coming out of a tree.
Information in this section was contributed by GlennII.

Secret areas:
There is a secret area in every level, Including the Ice Level and the Volcano level.
Information in this section was contributed by GlennII.

Hint: Easy Lives in Green Grove and Rusty Ruins:
In Green Grove Zone Act 1, collect all the rings. Then, when you move onto Act 2, collect 200 Rings and go through the hidden passageway before the third part of the stage. Collect the extra life, and complete part three. Then, when you go to Rusty Ruins, go through the little hole. SpinDash, and get an extra life. You should have your maximum of 10 lives.
Information in this section was contributed by SonicD3.

Hint: Final Fight Zone:
To unlock the final battle with Dr. Robotnik, you must collect all seven Chaos Emeralds.
Information in this section was contributed by Ralph.

Hint: Staying safe on spikes:
To stay safe on the spikes, such as the Spring Stadium, keep jumping. Every time you jump the spikes will retract, leaving safe ground to land then. As soon as you do, jump again and repeat the process.
Information in this section was contributed by Travis Schwipps.