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http://www.templateswise.com/
https://www.presentationmagazine.com/free_powerpoint_template.htm
I'd like to recommend Prezi (software or website). It's the brilliant alternative of PPT to present your seminar in a proffisioonal and different way. It is a free software for the essential features, and you can get a free Pro edition with educational account/email. It is quite comfortable, user friendly, and contain a lof of built-in templates to use.
