The arduino, the open source microcontroller board, has had MQTT support for a long time in the form of Nick O'Leary's arduino client. It does however require networking support which has traditionally provided by an add on shield, which increases the cost of the system.

The Nanode is an arduino compatible board which includes network support and can be built for approximately the same cost as a normal arduino board. It's still a work in progress, but is definitely worth a look if you want to use low power MQTT capable sensors/controllers.