Lik-Sang.com is having a sale on their Interact MegaMemory Manager Plus
here.
Apparently it uses compression to double the space on your memory card.
The main thing, though, is that it enables USB storage devices, much like that ungodly expensive X-Zip drive.
So you can connect a USB Zip drive or external hard drive to your PS2. I wonder if it works with USB pen drives.
Best thing is that it's only $5 (plus $7 shipping).