Hi Cap'n,
Sorry, I was sleeping ;-)
Rockwool is an inert material made by heat-treating volcanic
rock and spinning it out into fine threads. These are compacted
into slabs and are available from hydroponics shops in slabs
or various cube sizes. For sowing seeds they come in inch and
half-inch cubes with a small indent in for the seed.
The key point is that even when fully saturated with water they
manage to retain a large percentage of air - meaning that your
seeds won't rot and have the best of both moisture and oxygen,
just what it needs for successful germination.
Another plus is that there is no mess and the seedling can
be transferred to either a conventional pot with compost or
hydroponics rig without root disturbance.
Here's a tray of rockwool cubes: