My favorite daydream app idea lately has been one called “statements” or “beliefs”. Basically, you could use the app to make statements (like “I like chocolate”) that other people would either agree or disagree with. The only way I can think to make it viral is to have a leader board for the statements that recruit the most members. Also, you could divide your friends up (or be required to divide your friends up) as supporters or opposers to your statement before you are able to publish it. This was inspired by watching how Facebook users use groups to organize around a concept.
UPDATE: see also iThink.