mongodb
async - no more insane levels of nested indents when doing async javascript
express - middleware
ejs - templating system
logging - very simple logging module i wrote more than a year ago when there weren't good logging modules for Node yet.
knox - for storing doodles and static assets on S3
uglify-js - compress that client-side javascript
stylus - much nicer way to write css. variables, functions, mixins, etc.
everyauth - authentication - supports more than we could ever want but we're just exposing Facebook and Twitter for now.
useragent - we only support modern browsers, sorry IE users!
shortid - module I created to give us very short unique ids that are not guessable for chain sharing, like http://doodleordie.com/c/Eih5TDOHk or http://doodleordie.com/c/1hODyac94#1to4
moment - nice time references, like "7 days ago"
canvas - we render doodles on the server and then save those pngs instead of accepting pngs from the browser because people figured out they could send us any image they wanted.
ansi - doodles above a certain number of brush strokes we convert to ansi and echo in our console log. This is my favorite feature of the whole site, especially because I got my start in online communities in the 2400 Baud BBS world when ANSI art ruled.
We're using a 5 GB instance at MongoHQ with backups to S3.
Mongo is insane fast compared to what I'm used to with MySQL, and I love the JSON-based query syntax over SQL.
Not being able to do relational-type queries has made some things more difficult, like building "Top 10 List" of arbitrary data is a bit more tedious.
We have a lot of this information on our About Page but maybe that's not the best place for it.
I'll reply to each technology separately to make it easier to reply to questions about a specific tech.
This much ram has been perfect. We've actually had 1000 people playing at once on our first machine which only had 256 MB of RAM.
Sometimes I wish sites had something like a colophon that described how the site was made.