13,946 on 29 April this year - the day of the Royal Wedding.
The site for which I'm an Admin, theroyalforums.com, is a very large forum with an international membership that specialises in all things royal-related. We knew that for us the marriage of Prince William and Catherine Middleton was going to be bigger than Ben Hur and so had been planning for it for some months. In the event, all of the technical and administrative preparation paid off handsomely for our members and for the moderating team and the day went without a hitch.
31, immediately after tweeting about an article. Normal days have about 15 users online at the same time (which is a pretty decent score for a forum with 60 members I think, currently active at about 150-200 posts per day). Adding a chat has resulted in people staying longer.
980 is a record on a forum of mine. Regularly 300+ on a few forums. Got a new forum I launched in April and that's regularly geting 50's online when there are only just less than 500 members so a good 10% of the members every day. That's pretty good going IMO.