Emacs.

Just ensure you're running a 256-bit color terminal emulator, and for most uses it doesn't even differ that much from the GUI version. Recently I've been doing a lot of work by SSH-ing to my desktop machine from a small netbook and using the terminal Emacs client. If you run Emacs in server mode, the transition is so smooth that at this point, I don't bother getting out of bed if I need to work on something late in the evening, I just take the netbook and SSH over.