Because I’m a huge nerd and my google search results failed to bring up anything resembling a true listing, I thought I would throw together a little curl, grep and sed magic.
Mostly spent time adjusting the regex to get it just right and strip out all non-essential elements.
curl -s http://codex.wordpress.org/Function_Reference | grep -C 0 "title=\"Function Reference" | sed 's/.*:.*Function Reference\/.*">//' | sed 's/<.*>//' | sed 's/(.*)//' > wp-functions.txt
UPDATE: I was informed by Mr. Ryan Duff there is in fact a much more comprehensive listing over at WP Seek.