Doesn't depend on programming language but something with visual debugger. You know that stuff when you can see current line of your source code highlighted, press a key to step into, step over and so on. You can see values inside your variables. You can also change your variables mid-run right form the debugger.
Because you spend 20% of your time writing bugs and the other 80% debugging them. At least make it pleasant experience (no printf-style debugging).
Back in the day I was using Turbo Pascal, Delphi, Visual Basic, C#, Java, PHP with Zend, Java Script, today I'm using Visual C++.
You can always take a look how for example Windows 3.11 and earlier did it for their *.rtf file format and their "write.exe" editor / viewer / renderer (if you want to call it that way).