IoT OnOff®

A nice tutorial from Dan K.

A nice tutorial from Steve Cope

MQTT is a machine-to-machine (M2M)/"Internet of Things" connectivity protocol. It was designed as an extremely lightweight publish/subscribe messaging transport. It is useful for connections with remote locations where a small code footprint is required and/or network bandwidth is at a premium. For example, it has been used in sensors communicating to a broker via satellite link, over occasional dial-up connections with healthcare providers, and in a range of home automation and small device scenarios. It is also ideal for mobile applications because of its small size, low power usage, minimised data packets, and efficient distribution of information to one or many receivers 

HiveMQ Essentials gives a perfect in depth explanation of MQTT.

The MQTT Dashboard utilizes the HiveMQ MQTT broker. You can use any MQTT client or library to publish to the broker.

A nice online MQTT web client.

​A nice online MQTT web client with support for TLS

A reference to the Mosquitto test broker.

mqtt-spy is an open source utility intended to help you with monitoring activity on MQTT topics. It's been designed to deal with high volumes of messages, as well as occasional publications.