I just worked really hard getting rid of Singletons in the current project I am working on. They have a smell, but sometimes are very difficult to avoid.
My situation was that I had an API which constructed singleton objects for handling management functions such as Security, Configuration, etc. The API used EntityFramework and also pulled in PlugIns (using MEF). Because the construction of these classes was not always performed by my code (particularly for EF entities), it was not always possible to inject these into biz objects elegantly. So I used singletons, which can be accessed anywhere within a project subject tot heir scope.
