By far the best Passion site out there, dedicated primarily to the London production: Astrid’s Passion Page
Some basic information about the show: Passion page at Stephen Sondheim Stage
Another Sondheim site with a section for the show: Sondheim Guide/Passion
Another basic page about the show, with pictures Passion – by Stephen Sondheim
If you run, or know of, a page that should be added to this list, let me know!