ViImproved (VIM) is an improved version of the editor "vi", one of the standard text editors on UNIX systems. It has all the features you'll ever need from an editor, and probably three times that many more that you'll never use 
The newer versions also include a 'vimdiff' mode that you can use to diff and merge file(s). Oh, I didn't mention it's also scriptable in Python. Get it from http://www.vim.org/.