This paper describes the concept and architecture of geoXmart, a marketplace for proactive context-aware services, which enables users to easily find and subscribe to interesting services and allows providers to specify, export and maintain their services for versatile applications like electronic toll collect, context-aware advertising, or tourist information systems, even without implementing additional infrastructures. A geofence service comprises one or several geographic areas, so called geofences which are associated with actions to be performed, corresponding conditions and related metadata. Our approach defines a formalism of geofence services and their interfaces. Providers can benefit from simple service creation with almost no programming experience and even without the need to provide own infrastructure, because the services are executed mainly on the mobile device and are deployed on the marketplace. From a user's perspective, the concept provides ease of use e.g. finding services in their local surroundings and running serveral services in one single application, which leads to scale effects through combined processing of position and context. Therefore, we developed a concept for a marketplace providing geofence services that can be implemented by everyone, thus helping to increase the overall usage and integration of proactive location-based services in our daily lives.