Today, for your perusal and enjoyment, I give you three very different sources for free vector artwork:
First up, via Mordy Golding's excellent blog, is Go Media's Arsenal. Arsenal is heavy on skulls, zombies, horror, techno, etc. If that isn't your thing, scroll down further through their "vector packs", and you will see city skylines (including Minneapolis!), birds, lightning, amazing trees and more, all at very low cost. Visit their freebies page for a free sample pack of 42 pieces of art. Mordy has arranged a deal with Go Media for additional free art here. Go Media's blog is interesting reading as well.
Briar Press is on the total opposite end of the design spectrum. This is a community of printers and artists who are passionate about letterpress printing. The Cuts & Caps section of the Briar Press web site contains over 500 beautiful ornaments, initials, dingbats, and other artwork scanned from old specimen books and converted into vector line art. Note that these are free for non-commercial use governed by a Creative Commons license.
And finally, if you just need a "clip art" image of some random object, try the Open Clip Art Library. This is an "archive of user contributed clip art that can be freely used." Note that you can download a single package containing over 6000 pieces of art in the downloads section, or browse and keyword search thousands of newer images here. The quality of the artwork varies. Some is really nice, and some..., well, not so much. All the artwork is in SVG format. SVG files can be previewed and browsed in Bridge, and opened and edited in Illustrator, but will need to be re-saved as an .AI or .EPS file before placing in InDesign or other software for print use.