I just came across this website while searching for free PowerPoint templates. The site has hundreds of PowerPoint templates in many different categories including art, education, technology and business. Templates can also be searched for by tags, by doing a keyword search and even by color.

I found many of the templates to be of high quality and they included a wide variety of styles and designs from cute to casual to sophisticated. While not intended for printing, I believe many of them would also work well as backgrounds for posters printed on A4 or 8” x 11” sheets of paper.
Better yet, just like poster-street.com, there was no registration required to download the PowerPoint templates. You simply have to click on the ‘download’ link under each template (the site is rather ad-heavy, so be sure to click on the right button) and then save the zip file to your computer. Once you unzip or extract the file, you’ll see a folder similar to the screenshot below that contains all the graphics. Be sure to click on the file that ends with the .ppt extension.

Caution: I noticed the use of images of popular movie and game characters. I cannot vouch for how kosher this is and whether the templates follow special licensing agreements. Personally, I would stay away from using anything with Mickey Mouse or Mario just to be safe!