Cyclone is a low-level network toolkit that provides support for HTTP 1.1 in an API very similar to the one implemented by the Tornado Web server. It has localization based on gettext with pluralization support, native support for XMLRPC and JSONRPC, native support for WebSockets (drafts 75 and 76), decent support for sending plain text or HTML email with TLS and attachments, built-in support for Redis, and support for HTTP authentication.
TIXI is a fast and simple XML interface library for applications written in C, C++, Fortran, Java, and Python. Although simplified and somewhat restricted compared to a fully-fledged XML processing library, it can create documents, create and delete nodes, and add and remove element attributes. Routines for reading and writing text nodes and nodes holding integer and floating point numbers are included, along with routines that process aggregates of these simple types for the processing of geometric data, multidimensional arrays, or arrays of vectors.
Higgs.IO is a high performance, message oriented network library built for realtime systems. It provides a core extensible framework and libraries built on top of the core. Libraries include a WebSocket server, an HTTP server and client, and Boson, a custom serialization and RMI library.
Apache Qpid is a messaging broker that implements the latest AMQP specification, providing transaction management, queuing, distribution, security, management, clustering, federation, heterogeneous multi-platform support, and much more. It is extremely fast and aims to be 100% AMQP Compliant.