There area number of DLL's from Office 11 upwards that are required by MOSS to provide access to its full suite of functionality, from things like being able to edit lists in Datasheet view, to being able to upload multiple documents at once.
 
Here are the list of DLL's required for different units of functionality (stolen from Jose Barreto's blog - see: http://blogs.technet.com/josebda/archive/2007/03/26/searching-the-visible-moss-2007-code-for-activex-references.aspx)
 
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DiagramLauncher
Server Files: TEMPLATE\LAYOUTS\1033\BFORM.JS, TEMPLATE\LAYOUTS\1033\CORE.JS, TEMPLATE\LAYOUTS\1033\INIT.JS, TEMPLATE\LAYOUTS\1033\OWS.JS
Client File: DGRMLNCH.DLL
Function: Launch Microsoft Office Visio to open diagrams
Non-IE behavior: The browser will offer to save the file locally instead of launching the application. The option to "Open with Visio" will not show.

DigitalSignatures
Server Files: TEMPLATE\LAYOUTS\INC\1033\IntlCoreStrings.js, TEMPLATE\LAYOUTS\INC\Core.js, TEMPLATE\LAYOUTS\INC\intlcorestrings.js, TEMPLATE\LAYOUTS\SignatureDetails.FormServer.aspx
Function: Digital Signature for Microsoft Office clients
Non-IE behavior: You cannot sign documents, but you can view signatures.

ExportDatabase
Server Files: TEMPLATE\LAYOUTS\1033\CORE.JS, TEMPLATE\LAYOUTS\viewtype.aspx, TEMPLATE\LAYOUTS\1033\OWS.JS
Client File: OWSSUPP.DLL
Also Known As: SharePoint Export Database Launcher
Function: Export list to a Microsoft Office Access database
Non-IE behavior: Option to "Open with Access" will not show.

ListNet
Server Files: TEMPLATE\LAYOUTS\1033\CORE.JS, TEMPLATE\LAYOUTS\1033\CORE.JS, TEMPLATE\LAYOUTS\1033\OWS.JS, TEMPLATE\LAYOUTS\1033\OWS.JS
Client File: STSLIST.DLL
Also Known As: Microsoft Office List 12.0
Function: Edit list in Datasheet view
Reference:
http://msdn2.microsoft.com/en-us/library/ms416795.aspx
Non-IE behavior: The option to "Edit in Datasheet" will not show.

NameCtrl
Server Files: TEMPLATE\LAYOUTS\1033\INIT.JS, TEMPLATE\LAYOUTS\1033\OWS.JS
Client File: NAME.DLL
Also Known As: NameCtrl Class
Function: Display presence information for people, integrating with Outlook (mail, calendar) and Communicator (instant messaging)
Reference: http://msdn2.microsoft.com/en-us/library/ms455335.aspx
Non-IE behavior: Presence information will not show.

OISClientLauncher
Server Files: TEMPLATE\LAYOUTS\1033\IMGLIB.JS, TEMPLATE\LAYOUTS\upload.aspx
Client File: OISCTRL.DLL
Also Known As: OSIClientLauncher Class
Function: Integration with the Office Picture Manager
Reference: http://msdn2.microsoft.com/en-us/library/ms954038.aspx
Non-IE behavior: Integration with Picture Manager will not show.

OpenDocuments
Server Files: TEMPLATE\LAYOUTS\1033\HtmlEditor.js, TEMPLATE\XML\DOCICON.XML
Client File: OWSSUPP.DLL
Also Known As: SharePoint OpenDocuments Class
Function: Launch Office applications to open documents
Reference: http://msdn2.microsoft.com/en-us/library/ms454230.aspx
Non-IE behavior: The browser will offer to save the file locally instead of launching the application. The option to "Edit In (Microsoft Office application)" will not show.

OpenXMLDocuments
Server Files: TEMPLATE\LAYOUTS\1033\HtmlEditor.js, TEMPLATE\LAYOUTS\INC\Core.js, TEMPLATE\LAYOUTS\ctdmsettings.aspx, TEMPLATE\XML\DOCICON.XML
Client File: INLAUNCH.DLL
Also Known As: SharePointOpenXMLDocuments
Function: Launch InfoPath to open forms
Reference: http://msdn2.microsoft.com/en-us/library/ms452435.aspx
Non-IE behavior: The browser will offer to save the file locally instead of launching the application. The option to "Open with InfoPath" will not show.

PersonalSite
Server Files: None?
Client File: PORTAL~1.DLL
Also Known As: PersonalSite Class, PortalConnect.PersonalSite
Function: Adds your my site to a list of SharePoint sites maintained by the Office client
Non-IE behavior: No client-side knowledge of where your mysite is located.

Ppactivex
Server Files: TEMPLATE\CONTROLTEMPLATES\sldlibtemplates.ascx, TEMPLATE\FEATURES\SlideLibrary\SldLib\schema.xml, TEMPLATE\LAYOUTS\1033\SLDLIB.JS
Function: Integration with PowerPoint for Slide Library Templates
Non-IE behavior: The Slide Library integration with PowerPoint will not work.

RTEDialogHelper
Server Files: TEMPLATE\LAYOUTS\1033\BFORM.JS, TEMPLATE\LAYOUTS\1033\FONTDLG.HTM, TEMPLATE\LAYOUTS\1033\HtmlEditor.js
Client File: MSHTMLED.DLL
Also Known As: Rich Text Editor, DlgHelper, HtmlDldSafeHelper Class
Function: Rich Text Editor used to edit wikis, rich text web part
References: http://msdn2.microsoft.com/en-us/library/ms535238.aspx, http://msdn.microsoft.com/workshop/author/editing/tutorials/html_editor.asp
Non-IE behavior: A simpler multi-line text editor with no Rich Text capabilities will show. Consider using the Telerik control, which is available for free download at http://www.telerik.com/sharepoint and works with non-IE browsers.

SpreadsheetLauncher
Server Files: TEMPLATE\LAYOUTS\1033\BFORM.JS, TEMPLATE\LAYOUTS\1033\INIT.JS, TEMPLATE\LAYOUTS\NewTranslationManagement.aspx, TEMPLATE\LAYOUTS\lstsetng.aspx, TEMPLATE\LAYOUTS\new.aspx, TEMPLATE\LAYOUTS\sledit.aspx, TEMPLATE\LAYOUTS\slnew.aspx, TEMPLATE\LAYOUTS\1033\INIT.JS
Client Files: OWSSUPP.DLL, OWSCLT.DLL
Also Known As: SharePoint SpreadSheet Launcher
Function: Integration with Excel to import and export lists
Reference: http://msdn2.microsoft.com/en-us/library/ms427792.aspx
Non-IE behavior: Options to Export to Excel and Import Spreadsheet will not show. If you register an application to handle the .iqy extension, you might be able to export spreadsheets.

Stssync
Server Files: TEMPLATE\LAYOUTS\1033\BFORM.JS, TEMPLATE\LAYOUTS\1033\INIT.JS
Client File: OWSSUPP.DLL
Also Known As: SharePoint Stssynch Handler
Function: Connect to Outlook to synchronize lists
Reference: http://msdn2.microsoft.com/en-us/library/ms456225.aspx
Non-IE behavior: Option to "Connect to Outlook" will not show. You should be able register a client application to handle the stssync:// protocol.

UploadCtl
Server Files: TEMPLATE\CONTROLTEMPLATES\DefaultTemplates.ascx, TEMPLATE\LAYOUTS\1033\BFORM.JS, TEMPLATE\LAYOUTS\1033\INIT.JS, TEMPLATE\LAYOUTS\copy.aspx, TEMPLATE\LAYOUTS\updatecopies.aspx, TEMPLATE\LAYOUTS\upload.aspx, TEMPLATE\XML\FLDTYPES.XML,
Client File: STSUPLD.DLL
Also Known As: STSUpld UploadCtl Class
Function: Multiple File Upload
Reference: http://msdn2.microsoft.com/en-us/library/ms456628.aspx
Non-IE behavior: Only single file upload is available. Option for "Multiple File Upload" will not show.