npm-link(1) -- Symlink a package folder
npm link <folder>
This will link a source folder into npm's registry using a symlink, and then build it according to the package.json file in that folder's root. This is handy for installing your own stuff, so that you can work on it and test it iteratively without having to continually rebuild.
Linked Package Version
When linking a package folder, npm doesn't use the version in the package.json file. Instead, it creates a "fake" version number of:
"9999.0.0-LINK-" + hash(folder)
This way, linking the same folder will always result in the same version number, even if you bump the version in the package.json file. The extremely high major version ensures that it will always be considered the "highest" version, since it is a development bleeding-edge thing.