I like your question but these are plentiful. So I think I'll go with free PLUS how you could create your own.
As I read http://channel9.msdn.com/coding4fun/articles/Photo-Screensaver I see how we could, using free tools create exactly what we want.
Also in that article is the usual Picasa screensaver.
Do you have any suggestions for a photo screensaver that's free and operates easily?