In this paper, we present a web-based system, termed ISOGA, that uses isochrones to perform geographical reachability analysis. An isochrone in a spatial network covers all space points from where a query point is reachable within given time constraints. The core of the system builds an efficient algorithm for the computation of isochrones in multimodal spatial networks. By joining isochrones with other databases, various kinds of geospatial reachability analysis can be performed, such as how well is a city covered by public services or where to look for an apartment at moderate prices that is close to the working place. ISOGA adopts a service-oriented three-tier architecture and uses technologies that are compliant with OGC standards. We describe several application scenarios in urban and extra-urban areas, which show the applicability of the tool.