One of the key highlights of Web 2.0 trend is to engage user participation and enrich site with user provided content. The enormous valuable content for sites like Wikipedia comes from user participation. Wikipedia is online encyclopedia covering entire possibilities of content that’s available with us scattered around the world. User participation could also be narrowly focused on a specific site to bring more value to a specific area where users specialized or who have something interesting to share can add that value. I came across this site Radeopaedia.org which allows users to add content in the department of Radiology. I think there would be more such sites coming up focusing on specific areas and allowing users to add content when they have something useful to share to others.