GFork

Go to a tag on AO3, or build yourself a filter of works. This script adds a button (top right of the page, underneath the header).
Clicking it will pick a random work in your current view and redirect you there.

Now also available for your Marked For Later list (if you're logged in to your AO3 account).

I wrote this in a rush, so code's not pretty and this has not yet been properly tested. Feel free to ping me via Feedback if you run into issues.