I have been working on a client library for MQTT for the next release of Mosquitto. It is now at a stage where it is usable and ready for wider testing. There isn't any documentation yet (!) so it's only available in the source repository at http://bitbucket.org/oojah/mosquitto. Use the "get source" link in the top right corner of the page to download a snapshot. If you're interested in developing your own open source MQTT clients, it'd be great if you could take a look to make sure the interface is sane before I make a release!
The library itself is written in C, with bindings for C++ and Python.
I plan to package it up in a more easy to access form in the not too distant future, hopefully with some documentation as well.
Update
I've put the start of a man page online, which shows an example of using libmosquitto to subscribe to a topic and print the results: libmosquitto.3.