Recs.
Updated
Advanced Functional REST & SOAP Testing Tool.
With its easy-to-use graphical interface, and enterprise-class features, SoapUI allows you to easily and rapidly create and execute automated functional, regression, and load tests. In a single test environment, SoapUI provides complete test coverage - from SOAP and REST-based Web services, to JMS enterprise messaging layers, databases, Rich Internet Applications, and much more.
SpecsUpdate
Pros
Pro Cross-platform support
The experience on SoapUI on Linux and OSX is decent. You really can port your work.
Pro Service Simulation (Mocking)
SoapUI MockServices let you mimic - and create robust tests against - Web Services before they are implemented. They eliminate the expense of building full-scale replicas of your production systems and enable consumers to access the services without having to wait for them to be built or available. You can simulate any desired behavior, no matter how complex, and completely configure service responses.