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.
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.).
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.
jminix is a simple embeddable restful JMX console. It is useful for when you don't want to use an external full-blown JMX console, but just want to have a simple JMX entry point into your new or existing apps. Embedding JMiniX in a Web app is done simply by declaring a servlet. Deployed as a servlet, it benefits from your Web application configuration such as filters or security constraints.
Resty is a small, convenient API to talk to RESTful services from Java. Its focus is on simplicity and ease-of-use, often requiring only two lines of code to access RESTful Web services. It lets you use much less code than URLConnection, it's simpler than Apache HTTP client, and it's more reliable than Jersey client.