If you're looking at building a SFF PC, or perhaps some type of crazy mod, then you might not have the space for a conventional 3.5-inch hard drive. Transcend has launched a new drive that will certainly be small enough for you.
Their new 40-pin IDE Flash Memory Module is tiny, measuring 61.0mm x 26.9mm x 7.1mm. Surely you can find some room to squeeze one of these in. Granted, this is essentially a flash drive, but by making it compatible with a 40-pin IDE connection, they've taken out some of the hassle of installing your OS on a USB flash drive.
While this may not be the first of its kind, it is the largest. Transcend is offering the drives in sizes ranging from 32MB to 8GB.