Vim

I won't use Vim on Windows, but on *nix servers it really rocks. Takes a lot of time to learn, true. But surprisingly can be more efficient than GUI editors, when you get used to it.

I have been using Notepad++ for quite some time now, but had the feeling I have to try Vim out on Windows too, not just using it as a terminal editor. Day by day I find it more productive and fun then Notepad++. It's like changing from PHP to Ruby for me ;)

The steeper the learning curve the higher the reward!!

Learning vi opens your world to all sorts of things like vimterpreter in your browser or even vim in your shell/command prompt.

Just 5 years using vim and I'm still optimising my and time/work ratio by simply reading more on vim.
Once you "get it" you'll think twice about "normal" editors and even fancy IDE's. (vim could be one if u want, just read)

did I mention the steep learning curve ;)

vim just woks. It is far nicer the emacs but then again the emacs/vim war has been going on for ages ultimately it comes down to witch suites you best for me it's vim for you it might be emacs.

I like it for a perfectly simple reason - I can use the same editor (with same settings) between UNIX and windows.

Anybody who wants to work on a Unix or Unix-alike environment needs to learn vi/vim. It's not even slightly user-friendly:
* it has a bizarre modal structure;
* it has key bindings that are a bit of a stretch (h-j-k-l for left-down-up-right respectively, for example);
* its online help system assumes you know the odd terminology it uses and is thus doesn't help to learn the application;
* it doesn't play well with others in the GUI world (partially a result of the bizarre key bindings, but not entirely).

So why should you learn it? It's one of the only editors you're pretty much guaranteed will be on a Unix-alike. Any other editor (other than ed or ex -- don't ask!) is not guaranteed.

This was my favorite editor on Linux. It is seemingly infinitely customizable and extensible.