Here’s a really cool hack that shows you how to hack a PS2 controller onto your PSP.
Why would anyone want to do that?
The DualShock controls all of the PSP buttons (except for the volume and backilght) and can turn the PSP on and off. The right analog stick has been modified to copy the face buttons (like the razor nub), and L2 and R2 have been mapped to the left and right D-pad buttons (primarily for use in PS1 games, but useful in others as well). The analog On/Off button is used to power the PSP on and off, and the R3 button performs the Home function. All other buttons are self explanatory.