In addition to the "consumer-generated ads" marketplaces AdLab's covered, there are few others on this conveniently compiled list:
OpenAd.net: "the world's first online marketplace for buying and selling advertising, marketing and design ideas [that has] over 7,000 Creatives in 115 countries offering their work to a growing number of international clients."
Cambrian House is not advertising-specific, but the site's visitors have come up with some very relevant ideas, such as this one about rewarding popular MySpace users for promoting widgets among their friends on per-install or per-click basis (the Tom Sawyer effect).
Bright Idea has some bright advertising ideas. And you wouldn't believe how many Web 2.o idea-generating sites are out there that produce gems like ThesaurusSaurus.com, "the world's first social synonym website." Besides, you can always count on Halfbakery to come up with things like votable adverts.
Consumer-Generated XLNT Ads
Social Ad Creation at Zooppa
Vitrue to Mediate Consumer-Generated Ads
The Flip Side of Consumer-Generated Advertising
Advertising and Predictive Markets