Zipkin is great. It's pretty simple to set up, and you can instrument pretty much anything to send data to it. We made it a part of our internal httpclient and Kafka client so most teams get the tracing for free. You can even visualize the internals of Cassandra with zipkin. :)

There's support for a Spark-job that generates a visual map of all your services' dependencies[0] where the strength of their relationship is represented by the thickness of the lines.

[0]: http://thelastpickle.com/images/using-zipkin-graph.png