As said in the movie Robots, "see a need, fill a need," and that's exactly what Griffin did with iTrip for PlayStation Portable.
Though there are also iTrips for iPods, this one's a custom-fitting add-on FM transmitter that snaps into the bottom your PSP to act as its own little radio station broadcasting the audio out though the PSP's headphone jack to any nearby radio on an empty FM frequency of your choosing. It's powered by two AAA batteries of its own (so it doesn't bleed of the precious power off the PSP itself) which are graciously included, as is a soft carrying case that will accommodate the PSP with its new found height of the PSP plus 3/4", which is all the iTrip adds. The transmitter also includes pass-through jacks for the PSP's power adapter/charger and headphones.
The point of it all is to wirelessly share that otherwise personal listening experience of PSP audio on a radio, be it home sound system, boombox or car stereo. It makes for a pretty odd experience when your game (or UMD movie) audio sounds voluminously larger than the screen it's visualized upon, but it sure beats the clean but underwhelming sound of the onboard PSP speakers. Moreover, it's a real treat to have the mp3 player part of the PSP pumping out tunes for all to hear in the car, home or wherever.
While there are innumerable FM transmitters available from a variety of manufacturers that will do the same thing for just about any device with a headphones jack, iTrip is uniquely designed with the PSP form factor in mind, where it can sit simpatico and eternal as a bonus broadcast function of the PSP.
Of course, if you don't use your PSP as a music player and don't care to share your in-game audio with the whole household and half the neighborhood, then iTrip is pointless. Plus, at $70 CDN / $50 USD, you're near the price range of an entry-level mp3 players and an off-the-shelf FM transmitter combined, so you'll need to have a serious love on for your PSP to justify the iTrip, or disposable income, or both, in which case iTrip is pretty spiffy.