Ah, the plain old text editors - they can't be beaten. vi, Notepad (so simple it's bordering on stoopid), emacs and the rest. Quick, simple and essential.
Here's my pick for the best of the bunch:
1. jEditPretty useful as a plain text editor in the basic download form, but with the enormous suite of available plugins you can turn it into an advanced text editing tool, a full-blown IDE (for Java, PHP, perl, Python, Jython, C, C++, Ruby and more), a database control center or even... you won't guess this one... an accountancy program.
There are some great commercial tools out there (such as UltraEdit32), but this is more than a match for any of them. It's find and replace functionality is the best I've seen, and that is perhaps the most important thing for the manipulation of huge amounts of text, such as the 80000 line xml files I've had to work with.
Free and open source. Cross platform, some functionality not available on Macs.
2. UltraEdit (UltraEdit32)This is probably the only advanced text editor that can rival jEdit. For many people it's the ultimate choice and well worth paying for every year.
It has a mind boggling array of functions and pre-written macros for you to use and provides good code highlighting capabilities too.
Commercial, lifetime license available. Win32 only (still not ported for Linux - tut, tut).
3. Notepad2*A tiny Scintilla based editor (but without the ugliness), I actually prefer this to UltraEdit32, but it doesn't begin to compare as far as funtionality is concerned. It's just quick and simple, and that's what I love about it.
It's just a basic editor with some great functions, like move line up/down, change line endings (to Win, Unix, Mac) and code highlighting. It will replace Notepad for you and provide you with quite a powerful editor to boot.
Free. Win32 only.
4. ConTEXTThis is well established by now. ConTEXT will allow you to do pretty much anything, but where it falls down on is startup speed. Fully featured, but a little slow and unintuitive, ConTEXT isn't exactly my first choice, but it does provide a direct challenge to UE32, in that it follows the same lines.
Free. Win32 only.
5. PSPadI don't know what it is about all that great free software coming out of the Czech Republic, but this is seriously good. UltraEdit32 users will see a lot of familiar funtions here. Like all the previously mentioned tools, there is a file comparison feature that works very well. However, there are some quirky additions such as a Google Search on the current highlighted term, with advanced search available, and the choice to provide a line move feature with a difference ("swap lines" instead of move up/down).
I've put it in fifth place only because it's got a freeware look and feel as opposed to that open source feel, if you know what I mean.
Free. Win32 only.
Clever stuff all round - if you haven't tried at least one of these, it's time you did. Once you use them you'll wonder how you ever lived without them, especially if you have a lot of textual data to work with.
*
Notepad++ runs this one pretty close - it really comes down to my personal preferece