Public Domain, Creative Commons and Open Source
The terms Public Domain; Creative Commons and Open Source are defined briefly by clicking on them. They're worth being aware of. When you search for content to use in your presentations or creative works it would be best to start the search there. You have a great deal of freedom in what you do with them and don't have to worry about permission or any legal hassles if what you create and share become popular.
There are a few ways to easily find content that falls under these guidelines some of which will be noted here:
Advanced Google Search - Once you start your search click on the settings icon 
and scroll down. You can select content that is only marked for reuse.
The Internet Archive - This resource is full of content that is in the public domain, not to mention archived versions of webpages that might no longer be accessible.
SourceForge - Find, Create and Publish Open Source software. There are open source solutions for just about any kind of software you're looking for. Stop using illegal copies or paying for expensive licenses for programs when there is likely a better version available for free and legal use.
WikiMedia - It would be hard to avoid any of these resources these days. It includes Wikipedia, WikiSource, Wiktionary and more. While it's worth double-checking, almost all of these resources are in the Public Domain or are licensed under Creative Commons.
Works of the United States Government - Work prepared by an employee of the United States government is not subject to copyright. There are no restrictions for the reuse of its contents. More info on that here. That means that if you want to use statistics from the U.S. Census Bureau, the Bureau of Labor Statistics, FedStats, the CIA World Factbook or any other work produced by the government you can! It's still considered proper etiquette to cite your sources in a works cited page.