They make perfect small floral centerpieces, particularly if they have the shaker-style top. A local shop sells them quite regularly. Two methods:
First, buy fine sand or beads in various colours, fill the jars almost to the top. Put the shaker lid on top, and use it to position small silk flowers in a floral arrangement. Tie a coordinating gauze ribbon around the top, and position two or three in the center length of the table.
Second is to use glass-paint (available at Michaels, etc.), and paint a stained-glass effect on the outside of the spice jar. Fill with water, and position small real flowers - or silk flowers - in a floral arrangement. Tie a coordinating gauze ribbon, etc.
We have also re-used them in gift baskets. For example, a Movie basket gift, with DVD or movie rental gift certificates, popcorn and snack foods, etc. - purchase those popcorn flavourings at a local bulk food store, fill up the spice jars, label them with a funky label, and voila. Perfect gift. Same for home made bathsalts (or bathsalts bought in bulk). Or if you do baking and buy bulk "sprinkles" for decorating cookies.
I tend to use most of my extra spice jars for putting bulk spices in - as others have mentioned. I only buy small quantities of some spices/herbs because of their short shelf-life (6 months is recommendation), and I use these jars to store the small bags from the bulk food store. And I grow alot of herbs when possible, and when harvest time comes, I use these jars to store them (and share with neighbours).
Good luck, Shannon L.