Using a good text editor during development is very important, as it can ease the job. There are two powerful and old text editor: Vim and Emacs. They can handle large files, split window, indent smartly, highlight syntax, work in console (without GUI). However, both text editors have steep learning curve. Users have to memorize a lot of hotkeys if possible.

My primary text editor is jEdit. It also provides approximately unlimited window splitting. jEdit can also installed with multiple plugins. Users can also create their own macro script using Java language. And it is cross platform. Similar to jEdit, Kate also works with window splitting.