How to install a private NPM module without my own registry?

Can I just install an NPM package that sits on the local filesystem, or perhaps even from git?

Yes you can! From the docs https://docs.npmjs.com/cli/install

A package is:

• a) a folder containing a program described by a package.json file
• b) a gzipped tarball containing (a)
• c) a url that resolves to (b)
• d) a <name>@<version> that is published on the registry with (c)
• e) a <name>@<tag> that points to (d)
• f) a <name> that has a "latest" tag satisfying (e)
• g) a <git remote url> that resolves to (b)

Isn't npm brilliant?

cd somedir
npm install .

or

npm install path/to/somedir

somedir must contain the package.json inside it.

It knows about git too:

npm install git://github.com/visionmedia/express.git

In your private npm modules add

"private": true 

to your package.json

Then to reference the private module in another module, use this in your package.json

{
    "name": "myapp",
    "dependencies": {
        "private-repo": "git+ssh://[email protected]:myaccount/myprivate.git#v1.0.0",
    }
}