Updated: February 2nd, 2011
WordPress has a great way of letting you use simple text tags called shortcodes to provide a whole bunch of functionality, including custom PHP code. In this article, I'm assuming that you already know what shortcodes do and how they operate (if you don't, head over here: Shortcode_API).
One glaring omission in the way shortcodes are set up by default is that they only get triggered in the content of your post, leaving the sidebar and comments out. I'm sure this is done for security, so that your readers can't screw something up by posting shortcodes they're not supposed to – after all, shortcodes are PHP snippets on the backend.
However, let's assume you really know what you're doing …