All the free stock (Images are under the Creative Commons Zero (CC0) license: public domain. Check the specific licences for Videos, Sound Effects and Icons.)
Bioicons (Biomedical icons. Check licence terms for each icon, but most under Creative Commons Zero (CC0) license: public domain.)
Flaticon Free Vector Icons (Attribution required. Must attribute with "Designed by {authors name} from Flaticon".)
freepik (Free for non-commercial online use. Credit must be provided to FreePik. Free collection of vector images, PSD (Adobe Photoshop), icons and photos.)
iconmonstr (Can use for commercial purposes, can edit & change BUT no reselling of icons or claiming copyright on icons. Icons created
by Alexander Kahlkopf who retains copyright, but does not require attribution. Terms of use here.)
Noun Project (Wide range of icons and photographs. Check licence terms for each icon, but most under Creative Commons Zero (CC0) license: public domain.)
Emoji's on Pixabay (Creative commons zero. Copyright free and no attribution required - but avoid the images marked 'Shutterstock' in the top row. They are copyright and paid content.)
JoyPixels 5.0 Free (This set of icons is free for personal use or commercial testing purposes. You will need to create a free account. For more information see the Free License for JoyPixels 5.0 page.)
Clipart, graphics or cartoons
Creazilla (Collection of graphics or cartoons that are available under different licences. Some are under Creative commons. Others are used under the Creazilla open licence. This licence allows free use, including for non-commercial and commercial purposes. You do not need to ask permission or provide credit to Creazilla. Check each individual graphic for the appropriate licence.)
Open Clip Art Library (A large collection of clipart. Public domain, unlimited commercial use.)
Clipart Key (Clipart can be used for 'personal, non-commercial use'. This may include research and study and teaching normal degree courses)