Sharing and Forwarding
Seth Godin wrote a great piece on viral ideas. When Yahoo! acquired Yoyodyne (Seth's company) back in 1998 Seth did some talks for the production team. While I never had the pleasure of working with Seth directly, much of what he said in those talks has made its way into the Foneshow product.
We've worked hard to make Foneshows as viral as we can. Any user can forward any show to any other cell phone by simply pressing the 9 key. The person who has a show forwarded to them gets a message that their friend sent them a show (and the show name and length). They can listen right from their cell phone (usually just by hitting send). If they like it, they can subscribe from their handset with a single key press (without having to go to our website). Of course we didn't do enough to tell users about this feature until someone kicked us in the butt (thanks Jason, have fun in the Bay Area). Sometimes you're too close to the trees...
Also, go read our post on friction. If there's any significant friction, then your viral plans are dead.