(x)emacs
One way or the other I'm sure I've just made myself a pariah for mentioning I use emacs to vim users, or for leaving emacs until after vim in my list with emacs users. Still, I use both so I plan to discuss both.
xemacs or GNU emacs whichever you use are basically the same. xemacs is a fork of the emacs codebase, albeit a very old one. Personally, I use xemacs because many a moon ago it had fixes for some bugs I kept coming across. I know those memory leaks were all fixed a long time ago, I just haven't found a reason to switch back.
I think it would be justifiable to say that suggesting emacs is just an editor would be unfair to all the people who believe it is a lifestyle ;) Seriously though, instead of thinking about the normal editing related activities with emacs I plan to digress a little. Don't take this as a slight against the editing facilities mind you, I think out of all the editors I use on a regular basis the emacs variety is my absolute favourite.
The most obvious starting point as far as I'm concerned is emacs-chess, a wonderful example of not only the versatility of emacs but also of the lack of work my good friend Rach who introduced me to it does. emacs-chess is a frontend to the enormously powerful GNU chess application. When intermixed with a IRC session using zenirc in emacs for abusing the losing player it makes for a lot of fun.
Note
I'm going to break with my point here and point out vimirc, the vim almost equivalent. It is nowhere near as powerful or fast, but it is capable of providing you with a quick IRC session should the need arise when using vim.
You can also find out whether a game of emacs-chess is appropriate by checking the weather outside using the lisp weather tool. Which removes the requirement to look outside to see if the weather is suitable for exercise or a nice game of chess ;)
And when you need to look something up at google you can fire up w3 straight from within emacs, without changing desktops. Once you've found out some of the information you need why not put it in to a wiki for later reference?
Now I imagine some people might think I'm being a little factitious here, but I genuinely use emacs for all these tasks on top of text editing. I may not be as bad as Kenny who I remember saying "If I could use emacs as /sbin/init reliably, I would." If you use emacs and haven't tried some of the above packages you should, at least just to see the sort of barmy things you can achieve.
Warning
I imagine there are scripts for vim that provide almost all the functionality I've mentioned here, it is just that I haven't seen them. Feel free to drop me a line if you know of any vim scripts that provide the same functionality.
I think really my final point I want to make is that outside of all these extras you can use with emacs it probably provides every development related functionality you can imagine directly from the upstream package.