- nodetool will give you most of the basic metrics, such as read/write latency, heap usage, etc. It's very easy to use. http://wiki.apache.org/cassandra/NodeTool
- OpsCenter - a webapp built by DataStax.
Luckily, this isn't hard. You can query JMX directly, or you can query MX4J web interface that Cassandra exposes on top of JMX. I went with the latter approach, and wrote a simply utility that grabs all the available metrics out of MX4J and pipes them into Graphite.
You can checkout the source code and usage here: https://github.com/matvey14/cassandra-utils
You should be able to easily extend the script and push metrics to Ganglia instead by invoking gmetric, if you'd like.