DAX Studio

https://daxstudio.org/
DAX Studio is a free tool for running DAX queries against Excel PowerPivot models or Analysis Services Tabular models. It includes an Object Browser, query editing and execution, formula and measure editing and syntax highlighting.

DAX Formatter

https://www.daxformatter.com/
DAX Formatter is a free service that transform your raw DAX formulas into clean, beautiful and readable code. The syntax rules used improves the readability of the expressions. More details on Rules for DAX Code Formatting.

DAX Editor

https://www.sqlbi.com/tools/dax-editor/
DAX Editor is a Visual Studio extension that implements a language service for DAX language for SQL Server Analysis Services. You can edit the DAX measures of a project in a text file, format the code (optionally using the DAX Formatter service), and deploy changes to the original project and/or to a SSAS Tabular server.

Tabular Editor

https://www.sqlbi.com/tools/tabular-editor/
Tabular Editor is an editor alternative to SSDT for authoring Tabular models for Analysis Services even without a workspace server. You can edit a BIM file offline, without accessing to any data source. You lose any data preview, but you instantaneously change measures, calculated columns, display folders, perspectives, and translations.

DAX Patterns

https://www.daxpatterns.com/
DAX Patterns is a collection of ready-to-use data models and formulas in DAX. Each article describes one pattern in detail, including several use cases and sample files for both Excel 2010 and Excel 2013 (which you can easily import in a Visual Studio project for Analysis Services Tabular).

SSAS Tabular Translator

https://www.sqlbi.com/tools/ssas-tabular-translator/
Analysis Services 2016 and later versions have a feature to translate metadata in different languages. SSAS Tabular Translator is an editor for the JSON translation files exported and imported by SQL Server Data Tools for a Tabular model created in the new compatibility model (1200) for Analysis Services 2016.

VertiPaq Analyzer

https://www.sqlbi.com/tools/vertipaq-analyzer/
VertiPaq Analyzer provide internals information about memory used and compression rates of a model created in Analysis Services Tabular or Power BI.

BIDS Helper

https://bideveloperextensions.github.io/
BIDS Helper is a Visual Studio add-in that enhances development functionality in Business Intelligence Development Studio (BIDS) and SQL Server Data Tools (SSDTBI). It provides several features for a Tabular project that would not be available otherwise in the Visual Studio user interface: Actions Editor, Display Folders, Translations Editor, and HideMemberIf property. It also contains other useful features such as Unused Columns Report and Sync Descriptions.

BISM Normalizer

http://bism-normalizer.com/
BISM Normalizer manages Analysis Services tabular models, comparing different versions and merging differences in a safe way.

OLAP PivotTable Extensions

https://olappivottableextensions.github.io/
OLAP PivotTable Extensions is an Excel add-in that extends the functionality of PivotTables on Analysis Services models. OLAP PivotTable Extensions provides an interface for some of the functionalities available in the Excel API that are not exposed in the UI. It also allows to get the MDX query generated by the PivotTable in Excel to query an external or a PowerPivot data model.

Power Pivot Utilities

https://www.sqlbi.com/tools/power-pivot-utilities/
Power Pivot Utilities in an Excel add-in (based on a collection of VBA macros) that helps documenting PowerPivot models and facilitates the creation of pivot tables connected to the data model.

Workbook Size Optimizer

https://www.microsoft.com/en-us/download/details.aspx?id=38793
The workbook size optimizer is a tool that inspects the composition of the data model within a Power Pivot workbook, providing suggestions to reduce memory and improve compression.