I have had the privilege to introduce a few developers to Sitecore development.
Doing so I have run across a few tools that make life a lot easier for developers.
When dropping a .config file with application settings in the App_Config\Include folder it can be useful to verify that the changes are set in the Sitecore configuration. This can be done by using the showconfig page.
It can be located here in you solutions [Yoursite]/sitecore/admin/showconfig.aspx
Need to take a deeper look at the Sitecore database? Use the DBbrowser.
It can be located here in you solutions [Yoursite]/sitecore/admin/dbbrowser.aspx
Interested in knowing what Sitecore is caching and how much memory is used by it?
It can be located here in you solutions [Yoursite]/sitecore/admin/cache.aspx
This is a nifty tool made by Vlad Shpak.
This gives you access to all the common tools in Sitecore, as well as lets you set urls to custom tools such as Unicorn.
If you think I have missed a must have tool catch me at Twitter