There are lot of different resources around the Internet where you can download free PowerPoint templates and backgrounds for your presentations, but definitely one of the best places to find good PPT templates and other MS Office templates is in the official Office website.
In this website you can access the templates category page to browse the templates for a specific application. For example, if you want to download letterhead templates, business PowerPoint templates, abstract PPT templates and other topics you can browse this category.
Office.com was renewed for the new versions of Microsoft Office programs including new templates for PowerPoint 2013, Excel 2013 and Word 2013 that looks really nice and are powered with the new features available in the latest version of these products. So if you want to download free new PowerPoint templates with unique styles and backgrounds it can be a good source where to check for templates.
Download free Office templates from Office.com