BigQuery Select * Cost Guard is a Chrome add-on designed to help data engineers and analysts manage query costs in Google BigQuery. This tool is especially useful for preventing accidental high-cost queries that can occur when using the `SELECT *` command. It provides smart warnings and allows users to set a personalized cost threshold, ensuring that they are alerted before running expensive queries.

Key features include the ability to block the 'Run' button with a confirmation dialog if the query exceeds the set budget, and real-time cost flashing that highlights any potential expenses in red. Additionally, this tool emphasizes user privacy by operating entirely within the browser without sending data to external servers, making it a reliable choice for cost management in data querying.