I don't remember where I heard about it, but there's a hard disk monitoring utility that I've tried on on two of my computers.
It's very useful--it monitors drive temperature and SMART status. The temperature was something I was worried about with a new external SATA drive I had purchased a while back. I was using the 500GB drive to store and access video files during encoding for DVD authoring. So, getting the best performance from the drive without frying it was very important. Anyway, HDDlife let me monitor it to make sure temperatures were under control.
It can also remind you if remaining space gets too low--if you've ever run windows out of free space, it dies a very very slow painful death. HDDlife will warn you before you get to that point.
One strange thing is that it reports my external SATA drive twice. It shows up under Hard Drive 1 and Hard Drive 2 (in addition to the main system Drive 0). Strange, but it still works ok.
There is also a Google sidebar plug in that looks pretty cool.
Anyway, you can go and learn details about HDDlife. And yes, this is a (potentially) paid advertisement. But it's all still true.
EDIT: This isn't a paid advertisement...they never sent me the product for free. A mark against them. Woooo.