You could etch designs into them and hang them in a window

I have glass inserts left over from the exact same project. If you need more picture frames, you could use the glass for that purpose. Lots of the frames I find at garage sales and thrift stores do not have glass in them, so you can add the glass to a standard frame w/o glass and voila. It's not very creative, but it works. I'm still holding on to mine for a better use b/c I don't need any more picture frames.

I keep mine in a box because I now have lived long enough to know how the glass parts of picture frames can get broken. I think it's a little expensive to buy new glass for that purpose - and a pain in the butt.

You could use them as erase boards to write TO DO lists or reminder notes near the front door. A dry erase marker should work fine on glass.

Emily at Merrypad made terrariums out of them.

If you have a hobby shop with model trains take the glass and give it to them. Modelers use the glass from cheap picture frames for the glass windows on the train cars. Also microscope slides or the cover slips if you need some really thin glass for micro scale.

Are they sturdy enough that you could put some stick-on fabric or cork casters on one side and use them as trivets/coasters? You could even paint or attach a decorative paper to the bottom to make them more fancy.

I've placed clip art or other images I like on them, and trace the image with paints, then glue a felt or cloth backing and hang on the wall. It's great color, fun to make and cheap art!

use sharpies or paint and make faux stained glass art.