Japan's Kurouto Shikou insists that the 60GB hard drive found in the PS3 is too small. (It's from Japan, of course it's small.) To that end they've designed a SATA and eSATA slot for the system that lets you install third-party hard drives. You have to remove the built-in hard drive before installing the slot, but afterward your options are limited to however much you can spend on hard drives. (They're fairly cheap, by the way.)
Will the PS3 recognize all that extra storage space (unlike the Xbox 360, which can only access 20GB per hard drive, despite the latest hacks), and does it make sense to go to all that trouble to install a larger hard drive in the first place? Maybe if you're dumping Blu-ray movies left and right, otherwise, right now, this looks to be more trouble than it's worth.