Obviously, I would prefer that people do my Web-based experiments. Having done those, though, the best place to find a list of Web-based experiments is to check the list maintained at the University of Hanover.
Who is posting experiments?
One interesting question that can be answered by this list is who exactly does experiments online. I checked the list of recent experiments posted under the category of Cognition.
From June 1st through September 12, experiments were posted by
Brown University 2
UCLA 1
University College London 2
University of Cologne 1
Duke University 2
University of London 1
Harvard University 1
University of Saskatchewan 3
University of Leeds 2
University of Minnesota 1