Have you discovered Slideshare yet?
There are lots of slideshows in Slideshare which are free to use. Many can be useful in school assemblies, like this one on Wacky Inventions.
Direct link to this Slideshare Presentation http://www.slideshare.net/lucywads/50-wacky-inventions
AssemblyTube is looking for Slideshare shows, which could be useful in school assemblies. Here is the AssemblyTube KS3 Slideshare Section and the KS4 Slideshare Section.
If you find Slideshare shows which you think would be good to use in school assemblies, please let us know so that we can add them to our database. Then all assembly leaders can make use of them.