Today WiFi networks are widely deployed all over the world, with high Access Point (AP) density in urban areas. Both end-users and network operators are trying to exploit this density by combining multiple APs into "community networks," in order to share Internet connectivity, and possibly other services. WiFi networks suffer from decentralized management and a lack of coordination in frequency allocation and power control. In order to better use and configure these uncoordinated deployments, the first step is to characterize them. This paper presents a collaborative platform, where mobile stations collect simple network measurements (e.g., the presence of an AP) and send these measurements to a central system. By gathering and processing several network measurements from different mobile users, the platform characterizes the network deployment. This paper presents two applications: 1) minimal AP set, to reduce the energy needed to provide WiFi coverage in a given area, and 2) optimal scanning parameters, to minimize the time a mobile station needs for the network discovery. These two applications show the system's viability to solve particular problems of community networks.