An interesting free extension provided with OpenOffice.org 3.0 Beta is the Sun Presentation Minimizer. This is designed to reduce the size of files from OOO's Impress and Microsoft Powerpoint. It is for a delivery version of the presentation, which is not intended to be edited later. As a result it can reduce the resolution of images and then compresses them. Optionally it can delete speaker's notes, which don't take up much space but can be embarrassing if they weren't meant to be read by the audience.
Such a tool has been on my wish list for years. I find it very frustrating when organizing a conference and I get emailed multi megabyte Powerpoint files which have images in them hundreds of times the required size.
Unfortunately the results with the tool are disappointing. It only made a 50% reduction in file size on one presentation I tested. Set to JPEG the images showed too many artifacts to be usable. With lossless compression the colors looked odd. What I would like is an option which retains the image type of the original image (lossless or JPEG) while reducing their size. Also it would be useful if the tool looked for duplicate copies of the same image and replaced them with just one.