Kate is the default text editor in KDE, and also one of the most powerful and feature-rich text editors available for Linux. It can also be used successfully as an IDE (integrated development environment) and supports, among many others, spell-checking, highlighting for a huge amount of programming languages, it has an integrated terminal (which inherits Konsole's settings), encoding support. It supports sessions, plugins, encodings, bookmarks and even the possibility to split the current document horizontally or vertically. Kate is the complete text editor for any KDE user. A couple of months ago I wrote a full Kate review which you can read here.
Geany is a text editor with basic IDE capabilities which provides a good alternative to Gedit, the GNOME editor. It supports highlighting for various scripting and programming languages, indentation, and it provides enough options to configure it the way you want. I liked that Geany is well documented (via the Help menu).
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