Foundry offers a great selection of open source SVG illustrations, created by the designers at thoughtbot. Free to use on your next project.
Many Pixels offer royalty-free and customizable SVG illustrations, every week, to power up your projects.
Ouch! offers free vector illustrations for user interface, created by Icons8. These illustrations give a clean, trendy look to your designs.
DrawKit is a collection of free, beautiful, customisable MIT licensed SVG illustrations in two styles, to use on your next website, app, or project.