We're launching a new web site in July - a mix of social media and user generated content.
The site will have a broad and simple value proposition - think of this as horizontal segmentation. This will be complemented with multiple vertical niche segment building on the horizontal segmentation.
What is the one best idea you'd suggest to help us go viral?
If you have an advertising budget I would suggest StumbleUpon.com.
SU provides highly targeted traffic for example; bloggers between 18-35 who are searching for videos online. Not only would you be getting highly targeted visitors that may become long term users of your service but you are also introducing your product to the right crowd to help it go viral.
StumbleUpon charges $50 per thousand visitors which, though pricey, is a very good value for such targeted traffic.
Of course this all depends on the quality of the content and service that you are offering. No amount of money or planning can "make" something go viral.