We use a whole bunch, some of the main ones:

- async => helpful async library https://github.com/caolan/async

- nodeinspector => node js debugging in a nice webkit UI https://github.com/dannycoates/node-inspector

- riakjs => for talking to Riak: http://riakjs.org/

- knox => S3 helper https://github.com/LearnBoost/knox

- optimist => helper lib for writing command line node apps (no more bash :) https://github.com/substack/node-optimist

- mocha => unit testing node code https://github.com/visionmedia/mocha

- emailjs => email helper https://github.com/eleith/emailjs

- uglifyjs => code minification https://github.com/mishoo/UglifyJS