Very useful guide from Miško Hevery:  Writing Testable Code
Inside:

• Why Singletons, Registries are bad (also: Singletons are Pathological Liars)
• Why static classes should be used only as standalone functions containers
• How to use Dependency Injection (about LoD and more other)
• Where "new" keyword can be used
• and more other interesting

And good answer about using static classes at stackoverflow.com.