My “highly extensible editor” of choice is vim. Started using vi 10 years ago, at age 17, and I haven’t really looked back. I like to stay away from my mouse as much as possible.
I rarely work in compiled languages these days, so the only thing I feel like I’m missing from an IDE is syntax-completion. Constantly switching to a browser to look up the order of arguments to various PHP functions gets a bit tedious.
PS: I don’t want to start a vi/Emacs war here. I suspect that Emacs and vi[m] users have more in common with each other than either group has with IDErs.