Over the years, more cloud computing systems have been developed providing flexible interfaces for inter-cloud interaction. This work approaches the concept of inter-cloud by utilizing APIs, open source specifications and exposed interfaces from cloud platforms such as OpenStack, OpenNebula and others. Despite other works in the area of inter-cloud, that are mainly resource management-centric, we focus on designing and developing a service-centric architecture. We implement an inter-cloud bridge system that is elastic, easy to be upgraded and managed. We develop a prototype composed not only from heterogeneous cloud platforms but also from independent cloud services. These are developed by different cloud service providers and offered as open source Software as a Service (SaaS). The proposed Inter-Cloud Mediation Service uses Future Internet SaaS such as a Context Broker for registrations and subscriptions to services and a Complex Event Processing engine for event management. We present an experimental analysis to show interactions with various heterogeneous cloud platforms and we evaluate the performance of inter-cloud services separately and as a whole.