As far as I know (and I've been a harcore Excel user since 2.0), there is no way to stack tabs in Excel. There are some things you can do to help though. One of the easiest is to color code the tabs and maintain your tabs in Groups and then place the tabs in order within category. It's not what you are looking for but once you do this, you may find that simply Right Clicking the small scrolling windows of the worksheet. That will pop up a list of all the tabs in the workbook.
As if adding yet another tab to the group would be an answer

there is an app/script that you can add that automatically creates a Table of Contents listing the Tabs as hyperlinks (
http://www.mrexcel.com/archive/Edit/1059.html ).
You could also change the appearance of the tabs themselves - but its limited to width/font size etc... but perhaps a combination of changes to the appearance will be enough.