The concepts Social Software and Web 2.0 were coined to characterize a variety of (sometimes minimalist) services on the Web, which rely on social interactions to determine additions, annotations, or corrections from a multitude of potentially minor user contributions. Nonprofit, collaboration- centered projects such as the free encyclopedia Wikipedia belong to this class of services, as well as commercial applications that enable users to publish, classify, rate, and review objects of a certain content type. Examples for this class of content-centered Web 2.0 projects are del.iciou. us (for Web links), Digg.com (for news), Flickr (for images), and YouTube (for movies). Communication- centered services such as MySpace or XING enable individual communication and search for and within spatially distributed communities. So-called Web 2.0 mashups integrate and visualize the collected data and information in novel ways, unforeseen by the original content providers. The most prominent examples of mashups are based on Google Maps and overlay external content on a map. All these developments have a common approach of collecting metadata by making participation and contribution as easy and rewarding as possible.