Quite a few (over 10TB worth).
From memory I have:
One in the laptop (320 GB)
One in a 3.5" hot swap bay in the main computer (1x WD Caviar Black 2TB)
A 2.5" hot swap bay for the SSD (60GB as a boot drive)
Four external hard drives (1x Seagate Barracuda 2TB, 1x WD Caviar Green 2TB, 1x WD Caviar Green 1.5TB and 1x WD My Book 1TB)
And I bought another Seagate Barracuda 2TB hard drive last weekend.
I've not been having much luck with Western Digital HDDs lately (I've had two fail in the last four months), so I've decided to give Seagate a go.
I'm not that bothered about having the HDDs housed within the case since most of mine are in external enclosures. I only wished there was an easy way of getting more eSATA connections (I've got two on a PCI backplate, but I don't have any free slots available for more).