I'm an Emacs user. I love my editor (or OS, as some would think of it) because it has everything I need to do at my fingertips: no matter what I do, I hardly ever need to leave it, if at all. That way, I have familiar keybindings all over the place, it's scriptable and extensible, and documented better and more extensively than anything else I've ever seen.

I can browse a projects git history, and follow bugzilla links, for example (it's even possible to set things up so that different projects will lead to different bug trackers). I can also easily poke around in said history. I can find functions defined in a whole other area of the projects by using etags. I have an integrated debugger, which shows everything I need, and more. I have an email client with a ton of goodies (gnus threading is awesome, so is scoring and a lot of other things).

And I could continue virtually forever, but in the end, the reason I like Emacs is that it provides a familiar and integrated environment for every single task I can think of, and it lets me extend and enhance it at my heart's content.