Nanode, the popular arduino-with-ethernet board started early in 2011 is ideal for small MQTT based projects but has so far lacked an implementation of MQTT.

Nick O'Leary, the author of the original Arduino MQTT client, has created a Nanode implementation, but it isn't quite ready for the public.

Nicholas Humfrey has made public some code at that he says still needs some work but supports publishing QoS 0 messages of up to 127 bytes long and subscribing to topics with QoS 0.