Burp Proxy is an intercepting proxy server for security testing of web applications. It operates as a man-in-the-middle between your browser and the target application
soapUI, is the world leading Open Source Functional Testing tool for Web Service Testing. It supports multiple protocols such as SOAP, REST, HTTP, JMS, AMF and JDBC. soapUI is from eviware, the Open Source TestWare Company.
Varnish is a state-of-the-art, high-performance HTTP accelerator. It uses the advanced features in Linux 2.6, FreeBSD 6/7 and Solaris 10 to achieve its high performance.
Eclipse HTTP Client (HTTP4e) is an Eclipse plugin for making HTTP and RESTful calls. Build with user experience in mind, it simplifies the developer/QA job of testing Web Services, REST, JSON and HTTP. It is a useful tool for your daily job of HTTP header tampering and hacking.