• Japhar Java VM implementation
• Open Runtime Platform, Intel's research VM
• GCJ, Gnu's compiler + library + runtime
• Kaffe Java VM implementation
  
• OSKit, which (supposedly) can be used with Kaffe to create a standalone OS
• Chives Java VM
• Sun's JDK
• TYA, a JIT plugin for other VMs; the real download site is here
• Mozilla's ElectricalFire, a JIT
• Joust, a VM/OS
• The old Joust site
• Cacao
• Taurus VM for small processors
• Rockit, a VM by Appeal in Sweden
• IBM's VM for Linux
• Kissme, a free JVM
• SableVM, a free JVM

.java -> .class compilers

• Kopi, a GPL'd Java compiler
• Pizza compiler, with extensions to the Java language
• GJ (Generic Compiler) compiler, which was made by some of the Pizza developers
• Kiev compiler, with Prolog extensions to the Pizza language
• Guavac Java compiler / javac replacement

.java -> native static compilers

• Cygnus/GNU JC1 Java gcc front end
• Cygnus's CNI
• XDS Native Java

GC Theory/Implementation

• GC Papers archived by someone at UTexas
• Vern Martin's Garbage Collection In Java, and also here, and also some quotes

GC Action numbers

1. Normal GC
2. Finalization
3. Kill internal weak references
4. expand object or handle space
5. out of memory