XDate : gives you nice decorators over a vanilla Date object (which is pretty basic and outdated) including parsing, formatting and manipulation without all the headaches.
Normalize.css : Best of the CSS resets.
Modernizr: Good for targetting many platforms (especially non-PC eg. mobile and IPad – helps with feature detection)
JQuery + Bootstrap 2: This one is a no-brainer. Gives you responsive design (*-fluid), thoroughly tested and customisable and using Less or bootstrap-sass, can easily change styles globally. Becoming a defacto standard.
Underscore.js: I’ve found most people that have used Ruby like this library, it’s neat and powerful…