node.js utility library for working with objects and arrays?

underscore.js is a pretty good default for this kind of stuff. Here's a thread on compatibility issues that may be handy.

Edit, upon you're request for something beyond underscore:

As far as I know, underscore has become the defacto standard when you're looking for additional array operations (much like jQuery for DOM manipulation). Joyent maintains a pretty thorough manifest of node.js compatible modules, and the only seemingly comparable utility would appear to be an experimental library called fjs with an emphasis on currying (and judging from the source, most of the functionality comes from extending underscore functions anyway). There might be something else out there, but as far as I know nothing with the penetration and maturity of underscore.

Yet another edit - here are a handful of older libraries if you're so curious, but their maintenance has fallen off a bit - valentine, wu.js, Functional, and Sugar. Functional and valentine may be a bit thinner; wu.js looks to be about the same and sugar is even fatter.

lodash is a "drop-in replacement* for underscore.js" that you may also want to consider.

Lo-Dash v0.7.0 has been tested in at least Chrome 5-21, Firefox 1-15, IE 6-9, Opera 9.25-12, Safari 3-6, Node.js 0.4.8-0.8.8, Narwhal 0.3.2, RingoJS 0.8, and Rhino 1.7RC5

For extend specifically, you can use Node's built-in util._extend() function.

var
  extend = require('util')._extend,
  x = {a:1},
  y = extend({}, x);

Source code of Node's _extend function: https://github.com/joyent/node/blob/master/lib/util.js#L563