It's All Text! lets you edit text-entry areas (like LiveJournal post and comment fields) with your favourite real text editor. (So you can search-and-replace, load prepared files, and use whatever convenience features like abbreviations or filters your editor provides.) Works on Linux/Unix, and I gather it works on Windows; evidently it doesn’t work on MacOS X. "Remember, with great power outages come great responsibility outages."
Aardvark lets you interactively delete things from the page you’re looking at (or alternatively, select an element and delete everything else). It’s especially useful when you’re printing, because you can print out that one recipe that’s in the middle of a long string of comments, or print out the map and directions to the hotel without including the hotel’s banner, navigation sidebar, and list of reviews. Presumably you could also remove that one wide image that’s messing up your whole Friends page, although I haven’t had to try that since I installed it. It’s also useful for web developers or HTML/CSS geeks to see the structure of pages they’re looking at.