While external controller mods for the PlayStation Portable have been around for quite a while, someone from the Acidmods forums was able to come up with perhaps the cleanest looking handheld mod using a DualShock controller.
Acidmods member Blizzrad explained how the new mod made use of a single 25 pin connector mounted above the PSP power switch, which was compatible with a DualShock controller. You can hardly notice the mod at all; seeing how neatly tucked in the connector is on the side of the handheld.
Most of the buttons on the PSP have also been neatly mapped to the controller. The right analog stick simulated the face buttons (similar to razor nub), and the L2 and R2 shoulder buttons were mapped to the left and right D-pad buttons. You can even turn on and turn off your PSP with the analog button on the controller, while R3 allows you to perform the Home function.
If that's still not enough to impress you, you can watch a demo of the mod playing different games through the video below.