You probably know
npm list will give you a comprehensive report of the installed npm modules. The problem is you'll get the entire dependency-tree, while you're mostly interested in the top-level modules.
I've recently come across this quick way to list just the top-level modules, so I thought I should share it with you here:
npm list --depth=0
...or, if you want to list the globally-installed modules:
npm list --depth=0 -g