Viva - Open Source Java Runtimes

Kaffe
http://www.kaffe.org
a clean room implementation of the Java runtime (contains no Sun source code at all, and was developed without even looking at the Sun source code); licensed under the GNU General Public License (GPL).

gcj (GNU Compiler for Java)
http://gcc.gnu.org/java
a portable, optimizing, ahead-of-time compiler for Java; supports most of the core Java APIs including collections, networking, reflection, and serialization

Waba
http://www.wabasoft.com | http://sourceforge.net/projects/waba
small, efficient and reliable Java runtime for mobile devices; Waba for Palm OS; Waba for Newton; Waba for Gameboy Advance; Waba for MS-DOS; Waba for TI calculator; Waba for iPaq and more; licensed under GNU GPL

SuperWaba
http://www.superwaba.org | http://www.superwaba.com.br/vm.asp
extension of Waba; Virtual machine for Palm OS and Windows CE; licensed under GNU LGPL

SableVM
http://www.sablevm.org | http://sourceforge.net/projects/sablevm
a portable bytecode interpreter written in C; headed by Etienne Gagnon; licensed under GNU Lesser GPL (LGPL)

IKVM
http://weblog.ikvm.net | http://sourceforge.net/projects/ikvm
A Java virtual machine for Mono (.Net); licensed under a zlib/libpng-style license; headed by Jeroen Frijters

IBM Jikes Research Virtual Machine (RVM)
http://www.ibm.com/developerworks/oss/jikesrvm
Java runtime written in Java; Jikes is a self-bootstrapped virtual machine (that is, its Java code runs on itself, without requiring a second virtual machine); licensed under the CPL (Common Public License)

Intel Open Runtime Platform (ORP)
http://orp.sourceforge.net
Java runtime written in C/C++; licensed under the Intel Open Source License

joeq Virtual Machine
http://joeq.sourceforge.net | http://sourceforge.net/projects/joeq
Java runtime in Java; headed by John Whaley; licensed under GNU Lesser GPL (LGPL)

Kissme
http://kissme.sourceforge.net | http://sourceforge.net/projects/kissme
Java runtime; licensed under GNU GPL

Janos
http://www.cs.utah.edu/flux/janos/janosvm.html
Java VM that supports multiple, separate processes (called "teams" in JanosVM) within a single VM; research project spearheaded by University of Utah

Latte
http://latte.snu.ac.kr
Java runtime; spearheaded by Seoul National University; licensed under a BSD-style license

Aegis VM
http://aegisvm.sourceforge.net | http://sourceforge.net/projects/aegisvm

Jam VM
http://jamvm.sourceforge.net | http://sourceforge.net/projects/jamvm

Wonka
http://wonka.acunia.com
clean room Java runtime licensed under a BSD-style license

Japhar
http://www.japhar.org
clean room Java runtime licensed under GNU Library GPL (LGPL); last release on June 2002

Mono
http://www.go-mono.com
Open-Source Multi-Language Runtime; Supports C#, Visual Basic and More; Support for Java upcoming; headed by Miguel de Icaza

Portable.Net
http://www.southern-storm.com.au/portable_net.html
Open-Source Multi-Language Runtime in C; licensed under the GNU GPL; headed by Rhys Weatherley

Parrot
http://www.parrotcode.org
Open-Source Multi-Language Runtime; supports Perl 5 and Perl 6; support for Python, Java and more upcoming.

joeq Virtual Machine Links
http://joeq.sourceforge.net/other_os_java.htm

Kaffe Links
http://www.kaffe.org/links.shtml

Marco Schmidt's JVM Links
http://www.geocities.com/marcoschmidt.geo/jvm.html