With long articles, I love to have a Table of Contents nearly the beginning of the page, so that I can clearly see how an article/post is structured (if HTML heading elements are appropriately used), and I can also quickly jump to specific section of the document.
Previously with a few of my coding pages, I have to manually construct a table of contents, and ensure it stays synchronised with the actual content. Laziness in me calls for a plugin to be written to simplify the process, thus Table of Contents Generator WordPress plugin is created on a Saturday afternoon.
It basically scans through HTML headings in your post, constructs a Table of Contents, and replace every instance of <!--TOC--> with the actual TOC. It saves me a lot of time, and it is now easy to show how your page is structured!
Check the plugin page for more details.