Even Chris created his article of Useful Reference Books ages ago I just bumped into it, and after adding my bit I thought to do something similar in the field of development tools.
So feel free and add tools you found useful in your career - and check back to see what others are using.
No spam, no advertising, no commercial stuff. Feel free to add closed-source Tools, as long as they are available for free.
Integrated Development Environment (IDE)
Real Time Application Info
.NET (Sub) Frameworks
Disassemblers / decompilers / inspectors
XAML / WPF
Database related tools
Continuous build process / Build tools