Microsoft's answer to Adobe Flash is now shipping.
This cross platform (works on both Mac and Windows) browser plugin was built from the ground up with Rich Internet Applications (RIAs) in mind. Silverlight can also stream high resolution video (up to 720p), and promises to offer the same experience on both Macs and PCs.
If you want to create a Silverlight app, you will need to code in .Net.
For more information about Silverlight, and to download it, please visit http://www.microsoft.com/silverlight/
Silverlight works on Firefox and Safari on the Mac.