The Wayward Avatar's Walkthrough (Internet Archive) is well detailed.
Prescient Dragon's Complete Guide to Ultima IX is the best and most detailed walkthrough I've seen till now, though it's still under construction. It explains every detail of what there is to be done in each area.
Avocet Dragon's Ultima IX walkthrough can be found:
Hannibal's Ultima IX Page (Internet Archive) contains a walkthrough of some of the areas in Ultima IX.
Kuno's Ultima IX walkthrough can be found:
The Other Codex's Ultima IX Solution is incomplete and only covers Earth and Stonegate. A Hythloth walkthrough by Mythril Dragon is also hosted there (Internet Archive).
Quill Dragon's Lair hosts a slightly revised version of Ravage Dragon's complete walkthrough for Ultima IX. You may access it from the path of the Powerplayer.
There's a pretty good Ultima 9 walkthrough in point form at Mike's RPG Center. His Virtues Chart provides a quick reference about the virtues and their associated mantras, sigils, etc.
The Ultima IX: Ascension Prima Fast Track Guide is another pretty detailed walkthrough.
Gaming with Zack has a complete Let's Play Ultima IX Ascension video series on YouTube. After the playthrough videos, there are several others showing interesting things in the game.
DOUG the Eagle Dragon of IT-HE Software developed a video anti-walkthrough of Ultima 9 based on his past mischief in the game, consisting of the following parts:
