Most are free. 

Microsoft Excel Viewer

This program allows users who do not have Excel on their computers to view Excel files.  The viewer allows users to see the contents of a workbook, but no change any values or perform any calculations.  Download from Microsoft.

This add-in is the perfect tool to help you track down links in your workbooks.  It searches out links in formulas, defined names, and controls.  Download from Stephen Bullen's site.

This add-in allows you to remove menus created in Excel version 5 or 95 from your workbooks.  Once you've converted a workbook to version 97 or 2000, you can't remove menus created in an earlier version.  This add-in allows you to do this. Download from Stephen Bullen's site.

As you do extensive editing to a VBA project, lots of "garbage" gets left in the file because VBA doesn't properly clean up after itself.  Not only does this increase the file size, but it can lead to very strange behavior at run time.  CodeCleaner exports all your VBA code to text files, removes the modules from your project, and then imports the code from the text files.   This forces VBA to clean out all the garbage and start with a "clean" copy of the code.   I very strongly recommend this add-in.  Download from Rob Bovey's site.  (Or download the version for Excel5/95.)

When Excel97 was introduced, it replaced the existing Dialog Sheets with the far more powerful UserForm objects.  However, it didn't provide a way to convert existing dialog sheets to the new UserForms.  Rob's Dialog Converter automates this tedious and cumbersome task. Dowload from Rob Bovey's site.

This add-in will document the "vital statistics" of your VBA project.  Dowload from Rob Bovey's site. 

This is a collection of 25 routines that assist in working with defined names, worksheet settings, and range selections.  Dowload from Rob Bovey's site.

When you create your own command buttons on toolbars, you'll want to assign an image to the button.  Unfortunately, unless you know what image corresponds to what ID number, this isn't an easy task.  John's FaceID add-in displays each image, and its ID number, that you can use on a command button (and there are a lot more than you may think!).  Download from John Walkenbach's site.

Created by Bastien Mensink in the Netherlands,  this is a fantastic collection of over 300 utilities, all very well organized into a menu structure added to the Excel menu bar.  This is truly one of the best and most useful add-ins I've ever used.  Download from www.asap-utilities.com

This is one of the best add-ins for the Visual Basic Editor ever written.  It provides hundreds of tools to make coding easier.  Available free from www.mztools.com .

This is the only "for a fee" utility I list here. I was inclined not to list any non-free utilities, but PUP version 5 is so good and so useful, that I couldn't resist. (And no, I don't get anything for referrals from here.)  This is a truly remarkable set of tools.  PUP version 5 (works only in Excel 2000 or later -- not 97) costs US$40, or US$60 with the full VBA source code. See J-Walk & Associates PUP page  for more details and ordering information.

This free add-in makes is simple to search directly from within Excel the Excel newsgroups via Google.com for answers to questions stored in Google's vast archives of the usenet newsgroups. 

Download directly from Ron's web site.

The basic interface in Excel for managing defined names is, to put it charitably, lacking.  Excel MVP Jan Karel Pieterse has written a great add in for managing the defined names in a workbook. 
Download from http://www.jkp-ads.com/Download.htm#namemanager .

Smart Indenter by Stephen Bullen

This add-in properly indents VBA code for a structured and professional format. Download from www.oaltd.co.uk/Indenter

VBE Tools by Stephen Bullen

A variety of extension tools for the VBA Editor. Download from www.oaltd.co.uk/VBETools

VBA Code Compare

A very nice add-in that compares and manages changes in VBA Projects.
http://www.formulasoft.com/vba-code-compare.html

The bottom line:

For VBA, the best utility is MZTools, by Carlos Quintero.

For Excel, the best utility is ASAP Utilities (free) or Power Utility Pak (US$40).

Have you found other utilities that are "Must Have's" for Excel users?  Send me an email and I'll add it to the list.  The only requirement is that the add-in or workbook be free.