If you are interested in contributing to the code, we accept pull requests. Note: we have only just completed decoupling our Metamarkets-specific code from the code base and we took some short-cuts in interface design to make it happen. So, there are a number of interfaces that exist right now which are likely to be in flux. If you are embedding Druid in your system, it will be safest for the time being to only extend/implement interfaces that this wiki describes, as those are intended as stable (unless otherwise mentioned).
For issue tracking, we are using the github issue tracker. Please fill out an issue from the Issues tab on the github screen.
We also have a Libraries page that lists external libraries that people have created for working with Druid.