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Notepad++ is a plain text editor, primarily for programmers/coders, but if you work with ASCII tablature, it's a must-have.
It allows you to select/cut/copy/paste vertical chunks of text. This means you can add, move or delete bars of tab from the stave. It also lets you drag a vertical line from which you can then type on all selected lines simultaneously, allowing you to draw up bars of tab very quickly, like so:
I've yet to find another plain text editor with this functionality, but if you know of any others, please share!