Userscript-linkreligion is a project mainly written in JavaScript, it's free.
Adds or removes target=_blank on links
a) all external links should open in new windows.
I know, right?! I don't want to lose context just to go look at some other content for a minute! I'll load up target_external.user.js
b) links should always open in the current window unless I middle-click.
Tell me about it, target=_blank just leads to annoying window clutter I'll have to clean up later! I'll load up target_clear.user.js
Just drag your sacred user.js file onto your browser* and accept the confirmation prompts. As long as your browser is Chrome, or you have a Greasemonkey or equivalent extension installed...