I've seen an LED map light that plugs into and gets its power
from USB. It seemed to work OK.
I am wonderign why you'd want that. Personally, I find a PSU easier to
move around than a computer. If it's for use in a car, I'll either use
the cigar lighter, or add a power output socket somewhere sane.
I do curse IBM for not providing a +5V output on the printer port, though :-)
-tony