You could make picture frames with them. I have seen this in Mexican folk art--kind of along the lines of those hammered aluminum can mirror/picture frames. I'll post a link to a pic if I find one. You could make one helluva big mirror frame!
Update: I found these links. The first is to a animated gif that shows the progress of a bottle cap mosaic: http://www.kempa.com/blog/images/mosaic.gif The second is to a basket made with old--very old--bottle caps: http://www.folkartisans.com/images6/aokr1.jpg
And then there's something like this for the truly ambitious:



Good luck and have fun!