To view the best players screen (which is firstly seen on completion of the game), press B + Start at the title screen.
Flash Stab
Press Forward, Forward, Forward, B.
Flip Splash
Hold B, press Forward, Backward, Forward, and then release B.
Grand Spin
Hold B, rotate the D-Pad clockwise, and release B.
Instant weapon switch
To instantly switch from your Alt. Knife to the weapon currently selected in the weapons menu and vice versa without needing to open the weapons menu up, hold Mode and press B. Please note that this command requires a 6-button controller.
Secret Island
Go to the seashore on the lower west side of the map. Use Shade's Doppelganger to locate the invisible silver crack (the warp), then use your Warp Device to open up the warp.
You'll then be transported to a secret island located on upper west side of the map.
Note: This will only work after you have the Warp Device.
Sound Test
At the Diary (save select) screen, press B + Start to access the game's sound test screen.
Sword Flurry
With any sword equipped, 360 motion on the d-pad, forward, back, forward, B, pause for half second, B.
Triple Flip
360 motion on the D-pad, forward, back, forward, B, B, B (can also be ended after the 1st or 2nd flip).