Wikipedia has been built to gather encyclopedic knowledge using a collaborative social process that has proved its effectiveness. However, the workload required for raising the quality and increasing the coverage of Wikipedia is exhausting the community. Based on several participatory design sessions with active Wikipedia contributors (a.k.a. Wikipedians), we have collected a set of measures related to Wikipedia activity that, if available and visualized effectively, could spare a lot of monitoring time to these Wikipedians, allowing them to focus on quality and coverage of Wikipedia instead of spending their time navigating heavily to track vandals and copyright infringements. However, most of these measures cannot be computed on the fly using the available Wikipedia API. Therefore, we have designed an open architecture called WikiReactive to compute incrementally and maintain several aggregated measures on the French Wikipedia. This aggregated data is available as a Web Service and can be used to overlay information on Wikipedia articles through Wikipedia Skins or for new services for Wikipedians or people studying Wikipedia. This article describes the architecture, its performance and some of its uses.