This article provides non-exhaustive lists of Java SEJava virtual machines (JVMs). It does not include a large number of Java ME vendors. Note that Java EE runs on the standard Java SE JVM but that some vendors specialize in providing a modified JVM optimized for Java EE applications. A large amount of Java development work takes place on Windows, Solaris, Linux and FreeBSD, primarily with the Sun JVMs. Note the further complication of different 32-bit/64-bit varieties.
Azul Zulu — is an OpenJDK build supported by Azul Systems. It is open source and free to download. The initial release ran on Windows Server 2008 R2 and 2012 on the Windows Azure Cloud. Release in January 21, 2014 supports multiple versions of Linux as well as Amazon Web Services, Rackspace and various hypervisors. Also added an Enterprise version with subscription support available. In June 2014 Mac OS X support was added. 
Bck2Brwsr — small JVM capable to boot fast and run in 100% of modern browsers including those that have no special support for Java. Last release in September, 2013.
CACAO — uses GNU Classpath, supports multiple architectures. GPL. 1.6.0 released September 4, 2012.
GCJ the GCC Java compiler, that compiles either to bytecode or to native machine code. The product is currently in maintenance mode.
Azul Zing JVM a fully compliant Java Virtual Machine based on HotSpot that uses the Azul C4 (Continuously Concurrent Compacting Collector) garbage collector. Supports memory heaps of 100s of GB without GC pauses and is able to grow and shrink the heap based on load.
PERC (Aonix/Atego) is a real time Java for embedded. Aonix is not a licensee of Oracle.
SAPJVM (SAP) is a licensed and modified SUN JVM ported to all supported platforms of SAP NetWeaver, started as Java 5, in the meantime Java 6 compatible (Windows i386, x64, IA-64; Linux x86, IA-64, PowerPC; AIX PowerPC; HP-UX SPARC IA-64; Solaris SPARC x86-64; i5/OS PowerPC)
Waratek CloudVM for Java A multitenant JVM that allows per-tenant isolation, metering and resource control