Below, I am making a list of the tools I use most frequently for MOSS development, diagnosis and some maintainance.

SharePoint Manager 2007

http://www.codeplex.com/spm

This tool is a MOSS swiss army knife. It traverses the MOSS object model and displlay the properties for a vast number of MOSS objects. The tool also allows the user to make changes on the spot and commit the changes to MOSS. I can't emphasize enough how good this tool is.

U2 CAML Tool

http://www.u2u.info/Blogs/Patrick/Lists/Posts/Post.aspx?ID=1315
This tool makes building and testing CAML queries a simple task. It connects to MOSS and allows you query build live queries and test them accordingly.

Web Service Studio 2.0

http://mattharrah.com/blog/web-tools/net-web-service-studio-20/
This tool allows you view SharePoint through it web services. I use this tool for verifying data, extracting xml to verify and assimilate MOSS objects. Through it you can invoke webmethods interactively by providing a WSDL endpoint. The tool fetches the WSDL, generates .NET proxy from the WSDL and displays the list of methods available. Then you can invoke any method (with custom input parameters). The SOAP request is sent to the server and the response is displayed in the UI.

WSP Builder

http://www.codeplex.com/wspbuilder
This tool This tool is used for create web solution packages (wsp) from a folder structure. There is also a Visual Studio Add-in which allows you to build a wsp for a project at a time. This tool saves hours of configuration and testing per project. This tool is a must for building SharePoint solutions.

FireBug (for FireFox)

https://addons.mozilla.org/en-US/firefox/addon/1843

This tool integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page. It allows you to explore and modify the document object model (DOM) of a Web page. This includes the styles, style sheets and other resources. The Developer Toolbar can be pinned to the Firefox browser window or floated separately.

IE Developer Toolbar

http://www.microsoft.com/downloads/details.aspx?familyid=e59c3964-672d-4511-bb3e-2d5e1db91038&displaylang=en

This tool allows you to explore and modify the document object model (DOM) of a Web page. This includes the styles, style sheets and other resources. The Developer Toolbar can be pinned to the Internet Explorer browser window or floated separately.

I will add more tools as I remember and/or encounter them. I will use the MOSS Tools label.