It used to be the case that acquiring royalty free stock images or illustrations meant paying hundreds of dollars. Fortunately, thanks to the world of open source and public domain art and design, those times are behind us and there are now hundreds of thousands of high-quality images freely available for both personnel and commercial use. This is a list of some of my favourite sites for finding photos, illustrations and videos that are free to use for personal or professional projects.

Pexels
License: Free to use, attribution not required
Types: Photos, Videos
Pexels is a site where contributors upload their photos and videos to share with the community and the world at large. To encourage contributions Pexels runs regular challenges with cash or sponsored prizes. Their catalogue contains over two hundred thousand photos along with thousands of short video clips.
Any drawbacks?
Not really.

Artvee
License: Free to use, attribution not required
Types: Artworks, Illustrations
Artvee aggregates tens of thousands of scanned art and design images that have been made available by museums and libraries around the world including the Smithsonian and the Rijksmuseum. You can browse by category as well as search by artist or keyword.
Any drawbacks?
Copyright for public domain art is complicated and country specific, everything on artvee is public domain in the US and EU but not necessarily globally.

rawpixel Public Domain
License: Free to use, attribution not required (CC0)
Types: Artworks, Illustrations
Rawpixel is a marketplace for stock imagery and design resources who also provide a collection of public domain art and design resources like Artvee.
Any drawbacks?
Copyright for public domain art is complicated and country specific, everything on rawpixel Public Domain is public domain in the US and EU but not necessarily globally.
You must sign up for an account to download images.

Unsplash
License: Free to use, attribution not required
Types: Photos, 3D Renders
The elder statesperson of the open-source image world Unsplash boasts over two million photos contributed by the site's users.
Any drawbacks?
Not really.

Undraw
License: Free to use, attribution not required
Types: Vector Illustrations
Launched in 2017 Undraw is an incredible collection of more than 1200 vector illustrations single handedly created by the designer Katerina Limpitsouni. You can customise the colours of the illustration live on the website and download them in .svg or high resolution .png format.
Any drawbacks?
Nope.
ManyPixels Illustration Gallery
License: Free to use, attribution not required
Types: Vector Illustrations
ManyPixels is an on-demand graphic design service that also provide a free illustration gallery. You can customise the color of the illustrations and download them in .svg or .png format.
Any drawbacks?
Not really.

PixaBay
License: Free to use attribution not required
Types: Photos, Illustrations, Vector Illustrations, Videos, Music, Sound Effects
Pixabay is a community similar to Pexels and Unsplash where almost two and a half million images, videos, music and sound effects have been uploaded by creators around the world.
Any drawbacks?
You have to log in to download the highest resolution images.
DrawKit
License: Free to use, attribution not required
Types: Vector Illustrations, Icons
DrawKit is a project from designstripe that offers both free and premium collections of icons and vector images. Their free kits include more than two hundred illustrations and three hundred icons each available in 4 distinct colour styles.
Any drawbacks?
You must sign up for an account to download.
There are some frustrating prohibitions in the license e.g. downloads can't be used in web templates or app templates.

Pixeltrue - Free Illustrations
License: Free to use attribution not required
Types: Vector Illustrations, Vector Animations
Pixeltrue is a small commercial illustration team who also make more than five hundred illustrations and animations available for free.
Any drawbacks?
There are some frustrating prohibitions in the license e.g. downloads can't be used in web templates or app templates.