Stencyl - Game engine that requires no coding skills. A lot of extensions. Great user community. Free to build for web, reasonable subscription fee to build for PC, Mac, Android and iOS. Open source, annual subscription required for PC / Mac / Mobile builds.
Visual Studio 2015 - Debugging runtime crashes in Windows builds. Free.
hxScout - Performance optimisation tool for apps running on haxe. Free.
Diagrams.net - Powerful web-based diagrams editor with amazing UI and UX. Great for designing game logic. Open source.
Autodesk Sketchbook - Solid 2D drawing software with clutter-free UI. My primary graphics tool. Free.
Piskel - Robust pixel art editor with minimalistic UI and great UX, works in browser. Open source.
Krita - Powerful system for drawing and graphic design. Exports image layers to standalone files. Open source.
Inkscape - Powerful system for vector drawing and graphic design. Sets text to curved paths. Open source.
Free Sprite Sheet Packer - Robust online tool that instantly converts a set of image files to a spritesheet. Free.
Color Oracle - Colour blindness simulator. Great tool to improve accessbility of your game.
Color Brewer - Colour scheme designer with colour blind users in mind.
Audacity - Legendary sound editor with almost infinite capabilities, considering custom add-ons made by the community. Open source.
Open Broadcaster Software (OBS) - Screen video recorder with a good range of settings. Open source.
GifCam - Real-time GIF recorder. Quickly makes a GIF for a custom area on the screen. Free.
OpenShot - Capable video editor. Open source.
AutoHotkey - Tiny keyboard optimiser that uses a simple scripting language. Allows to only enable the Windows button when used in pair with another key. Accidentally pressing the Windows button on its own makes no effect.