The WordPress plugin I wrote, Category Posts in Custom Menu, supports custom extensions. Recently, I wrote an extension for someone who wanted to show all posts from some category, say “category A” that were ALSO in “category B” AND NOT in “category C”. So, I helped out (coding, yay) figuring it would give me an idea of whether the extension possibilities I created make sense.
If you’re interested, you can now find all the source code at GitHub. This extension was aptly (ahem) named “cpcm-and-and-not-extension”.