My favourite free tools for use with Microsoft SharePoint
ULS log viewer - Facilitates live viewing of all SharePoint logs farm wide, highly recommended.
DebugView - Lets you view the debug output from an application, can be useful when troubleshooting SharePoint webpart functionality.
DownloadWsp - As far as I know, there's no easy way to retrieve a solution file (.wsp) from the farm. However this neat tool, allows you to view and download any of the available installed solutions .
SharePoint Manager - Displays SharePoint in explorer view, helps to view the hierarchical structure of the farm.
WspBuilder - The best tool for building, deploying and upgrading SharePoint solutions from Visual Studio.
SPDisposeCheck - If you have not disposed of a SharePoint object correctly or even if you've disposed of the root web, which is not advised! Then this tool tells you. Can be used live in Visual Studio or even on a packaged solution.
Then a couple of useful toolkits for troubleshooting various issues: