Unit testing with singletons

Have a look at the Google Testing blog:

• Using dependency injection to avoid singletons
• Singletons are Pathological Liars
• Root Cause of Singletons
• Where have all the Singletons Gone?
• Clean Code Talks – Global State and Singletons
• Dependency Injection.

And also:

• Once Is Not Enough
• Performant Singletons

Finally, Misko Hevery wrote a guide on his blog: Writing Testable Code.