Silverlight
Milan Negovan points out that there are well-known usability issues with Flash - do we need another path to bad usability? Erwyn van der Meer has a great quote: "Just after text on the web has been semantically liberated from FONT and TABLE tags by judicious use of CSS, we are going back to the future...". He points out that at least the content won't be locked up in .swf files, so it could be indexed, which makes me wonder why Google hasn't tried indexing swf files. Maybe because there's not enough useful content to make it worthwhile?