Someone introduced me to Thinkature.com, and I have to say, compared to a lot of web whiteboarding software out there, it's not bad. Unlike almost every other web whiteboard I've tried, it actually works for me in Firefox, even with my NoScript plugin. The UI isn't amazing, but it does stay out of the way and let you draw. I like the ability to mix text cards, drawn figures, and images on the board. I really, really like the ability to have my own whiteboard and only the people I invite can draw on it. And it's free, too, though reg and a cookie are required. There are a few annoyances, but all in all, these guys seem to be doing web whiteboard way better than all the other places I've looked at.