Receiving and sending emails. Being an IRC/Jabber/others client. Displaying images and pdfs directly. Bulk editing of many lines, even from different files, at once. Drawing ASCII-art diagrams. Playing tetris. Being an amazingly powerful "calculator" (http://nullprogram.com/blog/2009/06/23/). Transparently editing file on remote machines, also executing commands on remote machines transparently. Formatting ASCII-art tables. Editing outlines/todos with support for semantic movement inside it (Org-Mode). Working in a terminal. Working in a background (--daemon mode) so that new instances just connect to the backend, reducing startup time to almost zero. Dumping a whole Emacs into single binary - all the memory Emacs uses is written to disk and next time you run it it starts very quickly, because you don't need to compile,, load and run any libraries; it's like suspend/hibernate for OSes. Having a decent mode for almost every file format out there. Displaying info about currently running OS processes, like top (proced). Multiple cursors. Rectangular selection and manipulation of rects. Registers for storing pieces of text, locations in files and others. Easy running of external commands, with or without capturing their output. Find&replace with an Elisp function as replacement (quick and effective way to transforms more complex files). Being a terminal emulator (you can run mc inside Emacs, although it's not the best possible experience). Being a shell (Eshell). Easy integration with external REPLs (like IPython, with colors and all). Displaying and working with IPython Notebooks. Splitting windows horizontally and vertically as many times as you want. If you made a bit too many windows (these are Emacs windows, they live inside one OS level window) you can enable tiling window manager for them. Easy integration with external linters for many languages. Displaying man and info pages. Having easily reachable docstrings for all the functions implemented in the editor. Hierarchical keymaps. Semantic editing of sexp (any lisp) code.

And probably much more - I still learn about some new, crazy useful feature at least once a week.

IDEs win in the autocompletion department and project navigation, but that depends on a language being used. For example editing Python with Jedi and Jedi.el (and Rope) in Emacs feels like working with full-featured IDE, with very accurate, context sensitive autocompletion, refactoring support, visual debugger and others.

emacs: incredibly customizable, you can re-write the way your editor works. Some features that I find important: editing files remotely over SSH, running a shell inside the editor (so the same editing commands work in the shell), easy to write custom file finding functions for searching different parts of a codebase, git integration (a semi-GUI interface to git), and if another editor has a feature, someone has probably created a package that adds that functionality to emacs. Also has tetris and pong.

http://stackoverflow.com/questions/1218390/what-is-your-most...