It's important to namespace your JavaScript code. I want one "master" namespace, and everything else under it (like YUI's namespacing approach). I fell into some potholes of browser compatibility on the way (with var foo; if (!foo) foo = {};), but here's what I ended up with:
// define master namespace
if (typeof foo === "undefined") var foo = {};
(function(){ // lexical scoping, allows "private" variables and functions
// define sub-namespace
if (!foo.bar) foo.bar = {};
// these don't leak outside
function helper() { /* ... */ } // "private" function
var counter = 0; // "private" variable
// this function shows up globally under namespace foo.bar
foo.bar.func1 = function(x) { /* ... */ };
// not just functions
foo.bar.constant1 = 12;
foo.bar.shared_data = [];
})(); // end of lexical scoping
This way all the JS files can use the same namespace hierarchy, and can still be defined separately. No JS library necessary.