The power of the OpenOffice2 file format
The Oasis document format is really cool. My wife who is an architect (the original house building version, that is) - had created a presentation in PowerPoint with a lot of images, and the resulting file was 100M+.
I opened that file in OpenOffice2 and saved it as an .odt file. Then I unzipped it, bulk opened all images in the Gimp and changed their resolution. Finally I zipped it all up, an the presentation was now 8M and looking just the same. Ah, the power of openess.