This popular tool can be used to prototype complete NDS projects without writing a single line of code.
The author of NDS Snoop has made numerous bug fixes to the original, and here's the new and improved version. For details, see this article.
NDS Snoop includes a README file with a detailed description of this tools and its components.
It now supports auxiliary classes, viewing data in hex and string format as well as selecting between multiple trees. The Readme file included with the original NDSSnoop is still accurate.