The WorldWide Telescope team has released a web client for WorldWide Telescope, written in Silverlight and currently in alpha but from my testing works quite well.
It doesn't support the 3D solar system view yet, but pretty much everything else is in there, including tours. Performance isn't as good compared to the full Windows client which makes use of 3D hardware acceleration. Nevertheless it is pretty useable, and the servers at the moment don't seem to be under as much load as they were when the full client was released last year. As someone who is in the process of making a couple of tours I can't grumble at the increased install base that having a web client will bring.
Check it out at worldwidetelescope.org/webclient.