From DZone: Understanding When to Use RabbitMQ or Apache Kafka. Short form: RabbitMQ is for lazy developers; AMQP message queues does most of the work for you at the cost of throughput (which is still high for most projects!), and Kafka does almost none of the work for you but can support much higher throughput. Your humble editor’s advice, earned by experience: if you’ve got the right developers and the time to put into it, Kafka can handle much more throughput than RabbitMQ can, but .. be prepared to make sure you have the right developers and the time to put into it. Otherwise, RabbitMQ (or equivalents) will do fine.
Druid is a high-performance, column-oriented, distributed data store. Haven’t tried it, it just looked interesting.
Not Java, but relevant: MP3 licensing has changed! The MP3 format is now available for use by pure open source applications. Woo! Now if we could get JPEG unencumbered in the same way…