- "Write once, run everywhere." as the sibling poster mentions. I can build on my Mac, deploy on whatever the server platform is.

Snapshot and release builds of our build server are distributed via a Nexus-managed repository. So, everyone codes against exactly the same dependencies, regardless of the platform.

- Hotswapping/JRebel.

- Good interoperability with other languages. E.g. the Typesafe folks implemented Akka in Scala. Java gets it for (well, almost) free.

- Easy monitoring and instrumentation.

Obviously, there are downsides as well, such as startup times (twofold: starting the VM and Hotspot detection/compilation), preset heap size, expensive JNI (native interface), etc.