If you are searching for a variety of content (images, audio, video, slides, etc.), the Creative Commons (CC) is an excellent place to start. The CC Search portal provides a front door for searching open content on a wide variety of popular sites, including Flickr, Google, and Wikimedia Commons. In addition, the Creative Commons hosts a list—built by CC-community members—of organizations and projects powered with Creative Commons licenses.
Wikimedia Commons is a media file repository of public domain and freely-licensed educational media content. In addition to a large image collection, Wikimedia Commons also features audio and video clips. Mashable is a blog focused on Web 2.0 and Social Media news. Mashable reviews new Web sites and services, and offers social media resources and guides. SlideShare is a business media site for sharing presentations, documents and pdfs on a wide variety of topics, which you can specify search on SlideShare to retrieve openly-licensed content. Scribd is an online platform for publishing and finding a wide variety of documents: school papers, poems, presentations, academic research articles, government documents, free online books, spreadsheets, opinion essays, and more. Scribd.com works like YouTube, but you browse and upload texts instead of videos.