UltraEdit ($50, 45-day free trial) has a well-deserved reputation for being one of the most feature-rich editors out there, and this newest version adds even more, mostly in the area of user interface and customization.
This program is a text editor, not a word processor. It is intended for the manipulation of raw text, not fonts, styles, and images. As such, the primary target is technical personnel of all sorts--programmers, manual writers, system administrators, and so on. For those in such areas, UltraEdit has all the tools one can imagine, and a few you probably never thought of before.
UltraEdit 15 expands on the already extensive list of features. For example, it is now trivial to turn a series of tabbed documents into a split-screen view, making it easier to do side-by-side editing and comparison. The plethora of windows, from a list of favorites to a list of functions in the current code segment, can be tabbed, docked, or set to auto-hide and auto-show, each individually. There are multiple environments, from "Power User" to "NotePad Replacement" which can quickly swap out windows and views depending on your needs. Syntax highlighting for most common programming languages (C, C++, Java, PERL, HTML, and many others) is included, and can be easily customized.
My only issue is that given the extraordinary feature set, the help file is a bit sparse, and I had to do a lot of experimenting to find out what particular functions did or what a given menu item meant.
Overall, UltraEdit is an amazingly deep text editing program. Even if you have one you already like, it's worth checking this out for the fully-functional--and generously long--trial.