Singleton <-> Static classes 

When singleton and not static classes:

• Create multiple singletons that start from the same class ()
• Instance returned by the Instance property could be an interface (the class can be an implementation of an interface)
• You can use singletons with interfaces just like any other class
  
• You can derived the singleton class (in order to extend the functionality)
• Unlike static classes, we can use singletons as parameters or objects

See also: http://coding.derkeiler.com/Archive/Java/comp.lang.java.programmer/2007-12/msg00697.html