how to make custom modules accessibly globally

I have written several custom nodejs modules and I want to be able to access them anywhere I run my node apps. I have tried placing them in the same directory I installed nodejs in, but that doesn't seem to work. Does anyone know how to do this?

Basically, this is what I want to be able to do:

`custom.js` 
exports.foo = function() {
     console.log("bar"); 
}

`app.js` 
var custom = require("custom"); 
custom.foo() //output: "bar"