Web services are a powerful tool for executing functionalities using third party applications and they are widely used in business processes. For this reason, it is necessary to complement traditional security solutions by adding soft security mechanisms, which take care of trust, in order to determine whether a web service and its provider are performing as they should. An extensible model for a trust evaluation system is presented for this purpose. It determines some parameters important in enterprise and Future Internet environments and it defines how to calculate the perceived trust by applying a three round algorithm with fuzzy logic. It exploits services semantics and it takes into account last updates about the service, consistency rules based on the semantic relationships between aspects and other specific calculations for each parameter. Semantics are used as well for sharing information with other platforms and federations for improving interoperability in distributed environments.