Umbraco UI is great and it's indeed possible to achieve much without a singe offline tool. However there are ofcourse some that makes Umbraco development more productive. This forum post is intended as a a step towards a wiki page with recommendations / suggestions in this matter, and I hope to get help with suggestions and information here. Please add info on wheter they are free or not + OS and what they help with.
Code editors
Visual Studio (win, free Express available)
NotePad++, (win, free) lightweight only text but with syntax highlightning
Textmate (iOS)
Source code versioning and repository hosting
Codeplex (free for open source projects)
Code editor extensions
ReSharper for Visual Studio (win). Refactoring help, code cleaner, intellisense improver.
Test driven development
Testdriven for Visual Studio (win) TDD
NUnit Quickstart (win open source)
Sql
Sql Management Studio (free Express available) create, edit, administer MS Sql databases
XSql Compare (win, free version available) compare databases
Xml Xlst
Visual Studio built in Xml/Xslt debugger
XSpec example using with Umbraco Xslts Unit testing Umbraco Xslts
Graphics
Photoshop
Adobe Fireworks
Paint.Net (win, free)
Mockups
Balsamiq (online, both free and paid)
Python
IronPython Tools for Visual Studio (win, free)
This is a companion discussion topic for the original entry at https://our.umbraco.com/forum/13234-great-offline-tools-to-have-in-your-umbraco-toolbox