Travelpod (the Travel blogging platform recently acquired by TripAdvisor - see CEO interview here) have put out a highly addictive Facebook application/game. In the game you have to race the clock to pinpoint where on the globe various cities and locations are.
It meets all the criteria for viral marketing apps - easy to use, fun, good but not excessive branding, easy to share and you keep wanting to go back to try one more time. In other words (unlike the recent Jetstar marketing effort) it keeps consumers happy and wanting more.
If you have a Facebook account (and are prepared to lose lots of time in addictive play), the go here to download the app. If you don't have a Facebook account then...well..go to Facebook.com and get one.
Thanks and curses to Kevin May at Travolution for sending me the app and condeming me to hours each evening searching for Bamako, Mali.