These are the development tools that I use. I will skip the obvious applications (which are typically the ones you pay for).

Applications:

• Reflector - Assembly browser and de-compiler for .NET components
• NDoc - Creates help documentation from XML comments
• Dependency Walker - Finds dependencies (great for non .Net app development)
• Process Explorer - Process management utility
• Resource Hacker - Great for modifying existing dll resources
• SQL Scripter - Generates scripts of an existing SQL Server database
• NSIS - Scriptable installer

Addins:

• Reflector addins - a list of addins for Reflector
• GhostDoc - XML comment generator
• CopySourceAsHtml - Converts source code to HTML.

Sites:

• PInvoke.Net - Converting API calls to managed methods