Stephen Colbert's Green Screen challenge -- where the talk show host asks fans to upload their own videos, based on footage of Colbert in a light sabre fight in front of a green screen -- should be inspiration for everyone trying to boost participation in an online community. Imagine a political candidate challenging voters to a green screen debate (risky! edgy! but sure to attract interest.) Imagine an advocacy group uploading green screen footage of an SUV (insteady of waiting for GM to set up its Tahoe ad campaign) and inviting people to make their own anti-SUV ads. Or imagine someone making a simple donation request in front of a green screen and inviting people to make it into a compelling video.

But why should the mash-up be limited to video? Let's think about audio mashups by providing an audio file (of a speech, a song, a debate). Or video collages by offering image files and online editing tools. Or make-you-own newsletters that offer a mix of Creative Commons licensed text and image and encouraging people to customize their own outreach campaigns using your online content.