For many applications (and their end users) the default MPS Project Explorer (aka Logical View) is too complicated -- it shows too much "unnecessary" stuff. Also, depending on the use case and the stakeholder, the structure of the primary navigation device must be different, adapted for the use case or stakeholder.
The platform contains a DSL to describe the structure of arbitrary alternative tree views in the project explorer; MPS already provides a drop down menu to select from various views, and the DSL can contribute additional ones.
The DSL supports the definition of arbitrary structures, labels, icons and actions, and also lets you add additional nodes useful for structuring the tree. As an example, we have built a new (and more flexible) favourites view. Below is a screenshot: