illegaladvertising.com is a blog on which you will find some of the best "underground" ads, from viral to street marketing, banned ads, spoof ads and so on.
The website does not limit itself to US ads so you will find a good selection of international campaigns. A small description next to the ad will sometimes give you further information about it.
They also have an RSS feed and Twitter for those who want to stay up to date.
Find them here: illegaladvertising.com