Apache ActiveMQ is an open source (Apache 2.0 licensed) message broker which fully implements the Java Message Service 1.1 (JMS)
RabbitMQ is an open-source message broker originally implementing the AMQP protocol with additional messaging protocols supported via a plug-in architecture
Sense easy worse
"What does rabbitmq do better or worse than activemq"
from question "Switching from ActiveMQ to RabbitMQ"
"It seems activemq is better than rabbitmq in the sense that it is easy to set up and maintenance for cluster"
from question "Are there any good alternatives to RabbitMQ for low-end setups?"
"Apache activemq which is way more popular than qpid or rabbitmq - or indeed any jms provider would work just fine"
from question "Serializing objects for asynchronous messaging"