Text Editors
Plain text editors are handy for quick edits. I especially like Textpad's "Find in Files" feature which makes it very easy to find things inside a directory and make a lot of changes when refactoring code.
CAD - Computer-Aided Design
If you are interested in designing enclosures or game pieces or other projects you intend to print with a 3D Printer then these are good tools for you to use to start to learn.