ATMT HD362-P External Hard Disk
 

Provided your needs are relatively simple, that is you want lots of storage at a great price and don't much care about fancy RAID arrays, redundancy, speed or compact size, then you simply can't beat a beefy hard disk hooked up to your PC by either USB2.0, Serial ATA or perhaps FireWire. In solely financial terms there's no cheaper way to add large amounts of storage space your PC without even opening the case.