QuoEdit is a plain text editor that runs on Mac OS 7.1 and later.
The most significant feature is probably that it supports AppleScript powerfully. Scriptable: full features of the application can be controlled by script. Recordable: you can record, replay and save your task sequence with/without the Script Editor (like keyboard macro). Attachable: you can attach your scripts to the menus (with some options) by either AppleScript or an included utility QuoEdit's Menu. To the customizable menus, you can attach not only AppleScript scripts but also text, tags and any files openable from the Finder.
Search, another significant feature, supports ordinary search and regular expression (regex) search. Batch find is also available.
Other features are, WorldScript (Mac encoding) and Unicode savvy, multiple levels of undo/redo, full shortcut keys, rectangular selection, and so forth. The text engine is its own code.
Incidentally, my favorite feature is that you can let the Finder open or reveal the target file (or folder) from file dialogs, by holding down Option (+ Shift) key. It's useful!