libfreenet is a Freenet library written in C. It provides client functionality, as well as a lower-level API for sending and receiving messages over a Freenet connection, and for reading and writing raw data streams.
To compile it, you'll need the OpenSSL library.
A test client is included, as is a small fragment of test server code. The test client is picky about URIs. You have to give a full "freenet:KSK@foo" URI, just "foo" won't work (yet). I've written up a little bit of ad-hoc documentation for it.