Getting Started
GOV.UK Frontend toolkit - A collection of Sass and JavaScript files for using as part of your application's frontend.
GOV.UK Templates - This provides a template containing the GOV.UK header and footer, and associated assets.
GOV.UK design patterns wiki - discuss design patterns for GOV.UK services.
Style Guides
GOV.UK elements guide - This guide explains how to make your service look consistent with the rest of GOV.UK.
Browser Testing and Accessibility
GOV.UK browser support guidelines - Service manual details
DWP - Device list and assistive technologies details
Prototyping
Most prototyping has been done using the GOV.UK prototyping tool kit.
This runs on Express.js which is a node framework.
This will get you up and running with the basics for a prototype for a service this includes many of the base elements (forms/texts) etc. and styles.
There are deployment guides available on the alphagov github account.
Software
For writing/editing code, most of our Interaction Designers and Front End Devs use either: