Update: Arbital Scrape V2
I've scraped http://arbital.com as the site is unusably slow and hard to search for me.
The scrape is locally browsable and plain HTML save for MathJax.
https://drive.google.com/open?id=1b7dKhOzfMpFwngAeI8efeOzv147Lv_mx
https://emma-borhanian.github.io/arbital-scrape/
If there's interest let me know as I may tidy up and open source my code. edit: working on this

Mirror: www.obormot.net/arbital
Thanks very much for this! I've written a lot of stuff on there (I'm the Patrick Stevens whose name is splatted all over the screenshot). I asked them a year ago (ish) whether I could have a data dump, and they said it was Too Difficult; and I didn't bother scraping it myself. I'm glad you actually went and did something about it!