SlideModel, SlideHunter, and FPPT offer slide design templates that we can use in PowerPoint, OpenOffice Impress, and Google Slides.
SlideHunter is a free website containing a collection of free slide decks. These slides can be downloaded then imported into PowerPoint, OpenOffice Impress, or Google Slides. Pro tip for Google Slides users, you can import all of the slides or select just the ones that have the layout you want. Once you've imported the slide templates you can modify them to fit your needs. And if you don't need complete slides, but just a piece of artwork or a diagram template, you can find those on SlideHunter too. I'm a big fan of this design that I found on SlideHunter.
FPPT is another site that offers free PowerPoint templates for teachers and students. You can search for templates on FPPT according to category, tag, or even color scheme. There is an education section on the site. In the education section I found this neat template for a history presentation.
SlideModel is a website that offers professionally designed PowerPoint slide templates. When converted, the templates also work in OpenOffice and Google Slides. You can register for a free account on SlideModel to receive their free template of the week. There is also an option to upgrade to plans with unlimited slide template access.
Disclosure: SlideModel is currently an advertiser on FreeTech4Teachers.com