Public Domain Photos contains both photos and clipart, and has several thousand of each that are in the public domain. While there are literally thousands to choose from, the search feature on the site isn't that great, and the organization of the images and clipart could be better. For example, if you do a search for 'wonton soup', it returns no results and only show professional images that link to shuttershock, which is subscription based. If you do a search on for 'wonton', it will return a few images from the free images on the site. If you can get past the hurdle of flipping through page after page within a specific category to find what you need, you will find tons of images that both you and your students can use, and lots of clipart options as well. All grade levels and all content areas can find useful images on this site, you just have to willing to sit and browse.
eRiding Media Library contains a collection of several thousand resources deemed copyright-free for educational use. It has 4 sections listed for available media: Images, Movies, Photographs, and Sounds - but some of the resources under images are duplicated under photographs. The organization of the images is a bit messy and hard to navigate but there are lots of pictures should you take the time to explore the site. I thought the Math section was interesting as it contained images of math related terms in a real world context, like angles or parallel lines to name a few. This section could be used for a variety of grades and I think it's a great resource for images students would likely use for presentations in Math class. The movies section currently contains Science related content regarding forces, which could be used for any grade level depending on the need. There are also sound files available; some in .wav format, and others in .mp3. Under sounds, there is a category for English Language - Phonemes which we thought would be useful for grades Pre-K through 1st.
PD Photo is a website that has a large collection of photos that are free and available for use under the public domain. There looks to hundreds of photos that fall under a variety of categories. Many photos are available for geography use and would be beneficial in a social studies classroom; there are also many images that can be used in a science classroom, like the Macros or Plants category. The images on this site are not limited to these two content areas as there are plenty of ways students and teachers can use these photos in an educational setting, for any and all grade levels.
This is a great resource for instrumental jams. I have used a few for animoto when I didn't want any lyrics, but wanted a certain beat or mood to go along with the images. It would be great for students making movies as well as the music is categorized i.e. blues, horror, dramatic, tense... and it allows a full preview before downloading along with a fairly large collection of free music.