Be able to group tabs the way Onenote does
By using a tab system the way Onenote works (with tabs showing vertically on the right hand or left and side) would significantly improve the usability of large workbooks, as you could group sheets into sections like, inputs, calculations, outputs, setup etc. It would also make it easier to see a listing of tabs rather than having to scroll along the bottom trying to find what you are looking for.
Michael Brown commented
For example, maybe you have ten stores in Ohio (which each has its own sheet) and ten stores in Minnesota and Wisconsin (for a total of thirty sheets). I would love to be able to click on an "Ohio" group where the tabs are now, which then opens vertically and allows me to select the proper sheet for the store I want. Scrolling back and forth across 30+ tabs is very inefficient.
Alternatively, tabs could be grouped or outlined in a similar manner to grouping rows and columns
This is similar to the Google docs spreadsheet function where you can click a menu and it will list all your tabs alphabetically so if you are looking for one specific tab you can get to it easily without having to scroll left and right. You can also search by typing the first letter of the tab name and jump to it. This is very helpful for navigating large spreadsheets of reference material.