The below links apply to to Power Pivot, PowerBI, SSAS Tabular and Power Query.
Dax Studio – This is a must have for anyone interested in developing Tabular models in Excel, PowerBI or SSAS. Some features include measure timing, measure formatting and easy execution of DMVs.
SSDT-BI – Tired of emailing 50mb Excel reports containing your model? Is company policy preventing the use of PowerBI? If so, then this is the tool for you! This tool provides for the creation of a server Tabular model, aka SSAS Tabular.
BIDS Helper – This tool is an add-in for SSDT-BI. Some features include multiple language support, display folders & unused columns report.
Workbook Size Optimizer – I’ve personally never been able to get this to work but it’s great in theory!
Excel File Cleaner – If Excel Inquire isn’t cleaning workbooks to your expectations then give this one a go. It’s light and very quick.
Tabular Translator (SQL Server 2016+) – This tool provides functionality similar to BIDS Helper but instead leverages existing functionality inherit in SQL Server 2016+
DAX Patterns – This website serves as a library resource for commonly used DAX measures.
Power Query Formulas – If this isn’t at the top of your favorites list then you’re doing something wrong. This is a must have for any Excel developer.
Dax Formatter – “A free tool by SQLBI that transforms your raw DAX formulas into clean, beautiful and readable code.” What more could you ask for?
Exporting an Excel Data Model to CSV – This is a nifty trick for exporting model tables greater than 1 million rows.