I've seen them used on crazy quilt squares. This woman has a terrific blog about doing crazy quilt squares and she uses a ton of buttons in this quilt. She shows how to do 40 some odd blocks. Plenty of ideas.
http://www.pintangle.com/journal/201...x-crazy-q.html
They can be used in the center of pincushions and pillows, top and bottom, to pull the center part in.