I like older engravings, so I tend to be rather unsatisfied with a lot of the free clip art I run into. But the free illustrations from the Clipart ETC website, an online service of Florida’s Educational Technology Clearinghouse (providing digital resources for the state’s schools), I like very much. The index of the 38,000 or so available pictures is here.
Students and teachers may use up to 50 clip art illustrations “in any non-commercial, educational project (report, presentation, display, website, etc.) without special permission”.