Feature Development on Web
Sublime Text 2 - A free text editor that has interoperable plugins with Text Mate. It also works flawlessly across Windows and Mac OS X.
SASS (Compass) - A easy-to-use language on top of CSS which gives developers the ability to create reusable mixins, css variables, as well as, auto concatenation and minification.
Feature Development on Mobile
Eclipse for Android Development - Necessary for any Android development.
Visual Studio 2010 - Necessary for any Windows Phone development.
Weinre - An easy way of debugging mobile-specific layout using WebKit inspector. An awesome project and drastically decreases the time an engineer spends on debugging.
Automation and Testing
Selenium UI - End-To-End UI testing using the C# SDK for Selenium UI. These tests can take a while to run especially when you have several core use cases to write tests against.
.NET HTTP Client - Our API endpoints are the backbone of our platform. Adding to that, we've invested heavily in server layer tests that use the .NET HTTP Client library. We specifically output JSON so we end up having to test passing parameters through HTTP POST/GET requests and assert the results.
Project Management and Source Control
Atlassian OnDemand (JIRA + Confluence + Tempo + FishEye + Crucible) - Atlassian provides an amazing suite of software development tools. We're particularly fond of the Task Board (swim lanes) in JIRA.
Subversion - We're currently on our way transitioning into using Git. Stay tuned!
GitHub (hackathons, open source, contracting out work) - GitHub is an extremely popular developer social network that provides a platform to easily collaborate on projects using Git.