Thanksfully, iPresentee recently released free Keynote Objects, a package of 100 images suitable for use in Apple Keynote, iWeb and Pages documents (as well as Microsoft Word and Powerpoint, for that matter).
The objects are resizable TIF images with transparent backgrounds, and include common icons like stars, flags, speech bubbles, signs, and computer-related symbols like folders and hard drives.
(Windows users: If you're having trouble converting, try downloading them here.)
Your slideshows and documents will never be the same. Thanks, Macworld!