Play as Mega Man without his Helmet
To play as Mega Man without his Helmet, hold shoot on Ryu on the stage select screen and then press Right Right Right Left. If entered correctly, a sound will play and Mega Man will lose his helmet on the stage select screen.
Hidden Abilities Code
At the title screen. Press and hold the jump and fire buttons for about 5-6 seconds. You'll have the ability to use Hadoken manually without any meter or defeating Ryu.
Pause the game in any level, then hold Jump (default L) and press U D D D (default keys W S S S) to have Guile's Theme replace the background music until you put the code in again.
More E Tanks
If you need more E tanks to beat the game. Just let the timer after you die hit 0, go back into the game and you will have all the tanks you previously collected and you can recollect the E tanks from the levels all over again. I got 10 E tanks at one point.
How to get an easy perfect
Just before killing your stage boss, use your E-tank to refill your health back to maximum. Doing this will trick the game into thinking you've earned a perfect in the round. Once you kill your boss when your health is full, you will earn a Perfect.