Mix-&-match illustrations of people with a design library, easily customizable. Free for commercial or personal use, created by Pablo Stanley.
Foundry offers a great selection of open source SVG illustrations, created by the designers at thoughtbot. Free to use on your next project.
DrawKit is a collection of free, beautiful, customisable MIT licensed SVG illustrations in two styles, to use on your next website, app, or project.
Ouch! offers free vector illustrations for user interface, created by Icons8. These illustrations give a clean, trendy look to your designs.