Freenet Libraries

There are a number of libraries in a variety of languages designed to allow you to conveniently write Freenet clients.

Java API

This is a link to a document written by Oskar Sandberg describing how to use Freenet's built-in Java API for writing clients in Java.

A Freenet client library in C : LibFreenet

Steven Hazel has written a C library designed to make it easier to implement Freenet clients in the C programming language.

ezFCPlib - easy yet powerful multi-platform C/C++ library

The ezFCPlib library was written by David McNab as part of the FreeWeb Freesite insertion tool, but has now been made available for authors of other Freenet clients.
ezFCPlib runs on Linux, native Windows (MSVC) and Windows with Cygwin and provides an intuitive, flexible and labor-saving way to interface with Freenet nodes (via the Freenet Client Protocol), for any combination of these platforms.
Documentation can be viewed here .
Another advantage of ezFCPlib is that no other third party software is required, which makes life especially easy for Windows client writers. ezFCPlib also supports both Freenet 0.3 and Freenet 0.4, and is actively maintained.