As gaming mods go, adding a mouse to the Sony PSP will either make perfect sense or seem just plain stupid, depending on the type of games you play and whether you do that near a table or not. Either way, it's a neat trick to fool the PSP (the USB port of which doesn't support USB-host) into recognizing the mouse; that's what one gamer has done.
Video demo after the cut
In actual fact, it's not USB at all that connecting the mouse to the PSP. Instead an old PS/2 mouse has been used, together with the PSP's serial port. A quick breadboard with the ATMEL ATmega8 chip allowed him to write up some code for the PSP; it eventually connects via the PSP Slim Remote.
All of the mouse buttons work, as does the scroll wheel, and there's even adjustable scaling and sample rates. You can map different PSP buttons to the mouse's controls, too. Again, the usefulness of this depends on what titles you play, but as a proof of concept it's pretty neat.