DAX Studio is an Excel Add-In for Power Pivot and Analysis Services Tabular projects that helps you writing and executing DAX queries.
It includes an Object Browser, query editing and execution, formula and measure editing and syntax highlighting. It will include integrated tracing and query execution breakdowns.
DAX Studio is a project on CodePlex – visit http://daxstudio.codeplex.com to read the full documentation, download the installer and the source code.
Paul te Braak published an introduction to DAX Studio on his blog.