Based on a couple of photos, it looks like the drive is a 3.5" desktop one. Assuming that's the case, all you'll need is... well, a case. Any 3.5" USB2 external case should work fine with it, and installing the hard drive into one is generally quite easy.
Go to Newegg or just look at whatever today's best deal is at dealmac.com (they have cheap external cases come up all the time). Although keep in mind that since it's only a 40GB drive, it's not worth spending much to get use out of it.
Also, there is a possibility that the drive has funky Sony-custom firmware on it that'll make it not work properly when installed in anything other than a PS2. If so, it'll be pretty much useless as anything else.
