Vitreous glass tiles - useful for borders. These are often used in traditional mosaics when no light will be transmitted but most colors are also beautiful when light shines through them.
All types of beads make wonderful additions to mosaic windows. I usually also put these in borders but they also work well incorporated into the image area. Look at some of the windows I have posted and see if you can identify beads.
Metal jewelry findings can also be used in mosaics. Remember when using them in glass on glass projects they are completely opaque.
Rhinestones - These are one of my favorite types of supplies to use. You can salvage them from old jewelry or buy them online. If the rhinestones have gold or silver backing they can be soaked overnight in Liquid Plumber to remove the backing. When attaching them to your glass be sure to glue them flat side down. Most of the time that will mean to use them upside down. If they are flat backs then you glue them right side up.