Kate is an advanced
simple text editor for the KDE environment, and offers some powerful features. For more simpler needs, KDE4 also comes with KWrite.
With support for some good features for working with multiple files, by allows multiple "tabs", an integrated file browser, file bookmarks, and sessions. With the sessions feature it is can save the state of the application, i.e. open files, enabled extensions, etc. which gives you a preliminary project-like support.
It also supports syntax highlighting, code folding, code suggestion, an integrated console, and even preliminary code completion, making it a powerful tool for writing or displaying scripts and codes.
With support for plugins, extensions and scripts, its functionality can be extended even further. Some of the functionality it supports such as spelling checking, text to speech are available using plugins included with the application.
For the advanced Windows user, this could be looked at as a replacement for applications such as Notepad (although it differs in functionality).