Standalone virtual machines don't come with the standard classes (the runtime library) which are typically stored in a file called rt.jar or classes.zip, so they are not “ready-to-use” as a Java runtime environment (JRE). It is sometimes recommended to use Classpath or the runtime library that comes with Sun's JREs.
Many of the projects in this section are results—or work in progress—of some research facility.
A 1.1 compatible virtual machine for RISC OS, also available for ARMLinux. Unfinished, but almost ready. Still, the page has not been updated since April 2001. Will not be free when it will be released.
http://www.mozilla.org/projects/ef/Seems to be gone.
A JVM with a JIT for the Mozilla project. Available for Linux x86 and Win32, designed to be easily portable to other platforms, distributed under the Netscape Public License. It's in a very early stage of development.
http://www.research.ibm.com/jalapeno/
A virtual machine for Java servers, written in Java. The project has recently become an open source project. The code is called Jikes Research Virtual Machine (RVM).
http://sourceforge.net/projects/ikvm/
A virtual machine for .NET. Available under the zlib/libpng license.
http://intel.com/research/mrl/orp/
Intel offers the Open Runtime Platform, which seems to be a set of tools including a just-in-time compiler, a virtual machine and a garbage collection module. The package on the download page includes C source code as well as precompiled Windows and Linux (x86 only, I guess) binaries. It is supposed to be used in combination with Classpath.org's runtime library. The tools are created by Intel's own Microprocessor Research Labs.
Very small virtual machine for PowerPC and i386 architectures, written in C. Currently interprets only. Distributed under the GPL.
http://www.cs.utah.edu/flux/janos/janosvm.html
A virtual machine based on Kaffe, distributed under the GPL. Supports multiple separate processes within a single VM.
http://www.hungry.com/old-hungry/products/japhar/
A portable VM distributed under the LGPL for FreeBSD (x86), NetBSD (x86, arm32), Linux (x86), Solaris (x86, sparc) and Win32. BeOS is not in the list of supported platforms, but there was something about BeOS patches in the News section.
http://www.eecg.toronto.edu/~doylep/jupiter/
A VM for Linux that is part of a research project. Jupiter puts emphasis on modularity and extensibility. Source code is available for download.
The GPL'd Kaffe virtual machine and its runtime library are a cleanroom implementation (no dependencies on Sun). There is a very impressive list of supported platforms: Embedded Linux, VxWorks, LynxOS, SMX, ThreadX, Linux (all distributions), DOS, Windows NT 4.0, Windows 98, Windows 2000, Windows CE, Solaris, SunOS, BSDI, FreeBSD, NetBSD, OSF/1, Unixware, HP-UX, NextStep, OpenStep. Supported processors include ix86, Sparc, m68k, StrongARM, MIPS, Alpha, PowerPC and PARisc. Memory footprint is small, the VM can be used for embedded systems. The exact Java version is not specified, for a reference see the Kaffe Datasheet (PDF, 96 KB).
KaffePC is a virtual machine for DOS, based on Kaffe.
There is also a port of Kaffe to AtheOS (no AWT available).
A GPL'd virtual machine, written in C (the JIT part in Java), targetted at Unix operating systems (main OS: Linux). Depends on Classpath as runtime library.
A virtual machine with a just-in-time compiler, available with source code under a "BSD-like license" for Solaris 2.5+ on UltraSPARCs. A class library is not included.
An LGPL'd virtual machine, written in C, using a modified version of Classpath as runtime library.
http://www.superwaba.com.br/en/overview.asp
Virtual machine for PalmOS and Windows CE. Distributed under the LGPL.
http://teaseme.sourceforge.net/
A virtual machine based on kissme.