Open Clipart is a long-running, community-uploaded clipart archive: currently around 186,000 pieces from more than 8,000 contributing artists.
The breadth is the point: whatever obscure object or icon you're after, there's a decent chance someone's already drawn it and uploaded it, even if quality and style vary wildly piece to piece.
Good for a quick, specific clipart search rather than for building a visually consistent set of illustrations.
Everything on the site is public domain (CC0) by policy, which sidesteps the mixed-license mess that usually comes with open community uploads: it's been running since 2004 and still takes new submissions daily.
Over 172,000 public domain graphics and clipart since 2004.
