Some of the Keynote themes come with a nice selection of shapes like Super Elements.
http://www.keynotethemepark.com/content/pages/superelements.html
I've also posted some files that contain Keynote shapes on several pages of my site at
http://www.makentosh.com. Some mirror some of the symbols Apple used to ship with Keynote, others are just fun, but you'll get an idea of what's possible.
http://web.mac.com/makentosh/tipsfromtheiceberg/Blog/Entries/2006/2/7Symbols_file_updated_for_Keynote3.html
ALSO, if you find some svg shapes you like, you can use the svg2key command (after you install it) to convert most any svg into a .key file.
http://mcb.berkeley.edu/labs/zusman/dave/svg2key/