The new paradigms that are gaining momentum in web applications empower users with a new role: users are no longer limited to consuming or creating online content, they also provide the semantic scaffolding holding together such content, thus taking on an active role in shaping the architecture of online information. The collaborative character underlying many Web 2.0 applications puts them in the spotlight of complex systems science, since the problem of linking the low-level scale of user behavior with the high-level scale of global applicative goals is a typical problem tackled by the science of complexity: understanding how an observed emergent structure arises from the activity and interaction of many globally uncoordinated agents. The large number of users involved, together with the fact that their activity is occurring on the web, provide for the first time a unique opportunity to monitor the "microscopic" behavior of users and link it to the high-level features of applications (for example the global properties of a folksonomy) by using formal tools and concepts from the science of complexity.
