A single file named `index.php` that loads a bunch of text files and spits out what looks vaguely like a blog.
It's a total mess. Business logic and HTML snippets scattered everywhere. Layers of caching to make the whole thing load in less than 0.03 seconds on cheap shared hosting. I'm sure I do a much better job for paying clients, but somehow never get around to fixing the decade-old garbage that runs my own site.
At least it doesn't have any SQL injection vulnerabilities :p