Below is an up-to-date list of useful tools every game developer should know about.
Finally, an extremely convenient way to share files over the internet, Dropbox is extremely helpful to developers who need to send prototypes to their testers.
Take screenshots in a flexible, fast-paced way.
SFXR is the simple tool we use to create sound effects. Even if you already know about SFXR, make sure you check out the AS3 port, SFXR-B, because it has more features.
You may want to collaborate with other people, and TeamViewer helps you do that. You can watch as your artist designs a character and offer feedback in realtime, for example.
This game engine is nothing short of amazing, but I do not use it, myself. If you are looking for a free 3d game engine with a WYSIWYG feel, Unity is your best option.
XN Resource Editor
You may not need this tool, but it could prove very useful in a bind. RPG Maker and Game Maker users will find this to be an invaluable resource. Get it?
List subject to change.