Sheet Navigator is a Google Spreadsheets add-on.
Table of contents for sheet, will open a sidebar listing of all the sheets in the spreadsheet. This allows for easier navigation of spreadsheets with many sheets.
Sheet Navigator creates a nice sidebar with a table of contents like listing of all the sheets in your spreadsheet. You can choose to toggle the visibility of hidden sheets, it will indicate whether the sheet is protected or hidden. Best of all it allows you to easily filter the sheets so that you are only looking at the ones you want to see. This tool is a must for spreadsheets with many sheets.