Earlier it was expected by Industry pundits that Sun in their JavaOne conference this week, Sun and Adobe will announce a version of Flash with Java Runtime to combat Microsoft's Silverlight. But Instead Sun has gone alone with their announcement of JavaFx which promises to give a Scripting Runtime, access to Java VM, 2D Graphics and more.
Adobe with its Flash, Flex and Apollo is clearly the incumbent with over 98% of installed base of Flash runtime. For Microsoft Silverlight is a huge step forward in this space and they have the advantage of bringing on board day 1 - millions of existing .NET Developers to Silverlight. Sun looks more as a late comer to the party. I am yet to study in detail on JavaFx - so I will hold on from making any technical comparisons at this stage.
If you look historically, what is happening now is clearly a re-run of the Browser Wars (to be precise Browser Based Applications war) between Microsoft with ActiveX and Sun with Java Applets. In the first round Java Applets got a slight majority, but Sun as a company didn't cash on it. Sun and Netscape let Microsoft eat from their hands royally. 
What was interesting was what happened later - Adobe (Macromedia) who never were in the platform business suddenly in the early 2000s became a dominant force with the ubiquitous of their Flash Player. Thanks to Microsoft who bundled Flash Runtime (probably without realizing how much reach it will have later) with every Windows/IE installation and there-by making Flash the de-facto plug-in. In the last few years, YouTube's of the world made Flash, a great Media Platform. Now Adobe wants to build on this huge advantage of Flash with Apollo and there-by make the Operating System (Windows) irrelevant - let us wait and see. Read here on what Bruce Chizen, CEO of Adobe has got to say on Microsoft's Silverlight. Interesting days ahead ...
Another entrant gaining ground in last few years is the camp of Web 2.0/Ajax with Google Map's and SalesForce's of the world trying to build everything with HTML/JavaScript. I have my doubts on the scalability of these Ajax solutions for serious business applications - even for simple effects the amount of lines of JavaScript and CSS you have to write is mind-boggling.
When I was writing this I was reminded of a presentation I made in 1997 titled "Building Browser based Applications (PPT)". Worth checking it out.
(The presentation is given as last edited on 22/11/1997 - My contacts and company logo are all out-dated)