The USB 2 Fujitsu portable drives are pretty quiet, mostly as they run at lower rpm than some of the latest Maxtor's et al. This lower rpm rate also saves battery power on my ibook & allows greater compatibility with respect to bus powered situations (my work Maxtor 7200 rpm portable will not power properly from my ibook). Nice case and build quality and useful travel case for drive & cable.
I have a 160GB 5400rpm Hitachi drive in an external 2.5" enclosure.
Its dead quiet, and makes a nice little backup disk. With the added bonus that its powered by USB/FireWire, and don't need an external power source.
Just got a Seagate FreeAgent desktop drive a few weeks back. There is cheaper options, but this one is very quiet, looks good, don't need to buy seperate enclosure, and best of all it has a 5 year warranty.