mechanize is a stateful HTTP/HTML client library written in Java to be available on the JVM. It utilizes Apache HttpClient for HTTP handling and JSoup for HTML parsing. Because mechanize is stateful, it will by default support cookies and hidden form parameters (like Rails CSRF protection). This enables client code to follow links and behave like a RESTful hypermedia client more easily.
SOAP/REST Mock Service is a simple configurable framework to mock out SOAP or RESTful (or "REST-like") Web services. Its main goal is to support automated integration testing of applications that integrate with 3rd party Web services. Functionality and usage scenarios of the mock service are in a way analogous to unit testing mocking tools (like Mockito or EasyMock). Once properly configured, SOAP/REST Mock Service exposes Web service mocks that can be called from application code and controlled via a REST interface from integration test code (setup, verify, etc.).
NanoHttpd is a lightweight HTTP server designed for embedding in other applications. It's only one Java file, in two "flavors", one strictly Java 1.1 compatible, and one at "current" standards. It supports GET, POST, PUT, HEAD and DELETE requests, and supports file uploading with very small memory overhead. Temp file usage and the threading model are easily customized.
RoboZombie allows easy integration with remote services by allowing you to replicate an endpoint contract and generate a proxy to access it. Contracts can be very flexible in terms of the resources they access. These could be vary from static HTML content or an RRS feed, to a RESTful web service endpoint. Each endpoint contract is specified on a single interface using annotations to provide the communication metadata. It is then wired into your code via an annotation, where it is created, cached, and injected at runtime.