Userpics is a free Craftwork pack of 100 avatar illustrations, "friendly heroes" in the site's own words, delivered in both circle and square crops for direct use in user lists or profile mockups.
Having both crop shapes ready-made saves the step of cropping avatars yourself for whatever UI pattern you're using.
Fits the same avatar-placeholder job as UI Faces, but in an illustrated rather than photorealistic style.
Free and commercially usable, SVG and PNG, with Craftwork's paid Pro Access subscription available separately for their broader library.
