Billboards that listen to the conversations of people in their vicinity and displaying all the ads that fit to show, now in Second Life:
"A ContextAds board listens to the conversations of those avatars around it, displaying advertisements when certain keywords are mentioned. Advertisers can bid on keywords, with their account only being charged when their ads are actually shown. Avatars can click on advertisements of interest to them, and can be offered a website, or a teleport to an in-world location."
Not a new idea at its core -- Google shows contextual ads in GMail and there were rumors that MSN messenger was displaying ads based on what chatters typed in -- but I love the Second Life context.
New World Notes has a video. I don't know if the VW Tuareg billboard is spec work or was actually commissioned by the company (probably the former).
Earlier and in real life:
MINI Tests Personalized Billboards
Google Eyes Contextual Billboards
Billboards With Face Recognition from Microsoft
AdAge on Contextual Billboards (with more links to relevant posts)