Interested in load testing your REST or SOAP APIs? SoapUI Open Source and LoadUI NG Pro in Ready! API can both be used to do varying degrees of load testing. The choice of one or the other really depends on what you're trying to accomplish and how much time you have.
SoapUI Open Source: If you have time on your handsand only need to do a quick concurrency check, SoapUI Open Source may suffice. It provides basic stats and is constrained to a limited amount of load based on hardware and network configuration.
LoadUI NG Pro in Ready! API: For professionals and businesses that really need to be sure about the performance and scalability of your APIs, there's LoadUI NG Pro which includes distributed load testing, server monitoring, visualizations, and advanced scenario simulation.
SoapUI Open Source