Considering he was a major force driving the rewrite of Reddit from Lisp (the version that HN is based on currently) to Python, adding features like Markdown support (a language he helped create in 2002), Aaron defined major parts of the site that have persisted across multiple rewrites of Reddit's codebase.
Hacker News is written in arc. The first reddit was written in Common Lisp. clisp, specifically, I believe.
Aaron convinced them to do it in his web.py and it was rewritten in a weekend (or so the story went). Later, it was written in Pylons and open sourced. No idea these days.
This was pretty common back then even for extremely complicated (for the time) sites (graybeard here). FCGI rewarded having a single huge application that would gradually leak memory over the course of the day until you told the intern to bounce the service when certain red lights started going off too frequently.
