Free PowerPoints

Pete's Power Points is a low-budget site that hosts a collection of PowerPoint presentations in almost any subject. Most of these are created by teachers, and some are better quality than others. Its potentially a good starting point if you need to plan a PowerPoint for a class. They're all downloadable, so you can find one you like and make it your own. 

Kathy Schrock's Guide for Educators

Kathy Schrock's site is really a collection of links, though there are a few articles she's written herself. There's not a lot of quality control determining what links are used, so you should have something specific in mind when you use this site and be ready to filter through stuff on your own.