What are the good open source implementations of Java Virtual Machine?

java, jvm, open-source

Solution

Hotspot (GPL, by Sun)

Harmony (Apache, also the basis for Android's Dalvik VM)

JamVM

CLDC - Connected Limited Device Configuration(Sun again) This one is not really open source, but the source code is available.

Problem

I am trying to write my own JVM. Can you point me to some existing open source implementations. Thanks.

Original source