Apache HttpComponents Client
The HttpClient component supports the client-side of RFC 1945 (HTTP/1.0) and RFC 2616 (HTTP/1.1) , several related specifications (RFC 2109 (Cookies) , RFC 2617 (HTTP Authentication) , etc.), and provides a framework by which new request types (methods) or HTTP extensions can be created easily.
Apache HttpComponents Core (blocking I/O)
Async Http Client library purpose is to allow Java applications to easily execute HTTP requests and asynchronously process the HTTP responses.
Apache HttpComponents Client fluent API
Apache HttpComponents AsyncClient
Jetty :: Asynchronous HTTP Client
Google HTTP Client Library for Java. Functionality that works on all supported Java platforms, including Java 5 (or higher) desktop (SE) and web (EE), Android, and Google App Engine.