There's one made by Datel, but I've heard it's not worth it. I've also heard the Datel "hard drive" is nothing more than a CF card that you can put in memory stick duo slots.
The prices of memory sticks are falling (4 gb as low as $110.) I have heard of devices that have like 40 gb that plug into the PSP, but you can't access 40 gb at one time. With those devices you would still have to have a big memory stick.