SilverLight's rival is seeing the light
Flex Builder Alpha has been released for GNU/Linux, Flex is Adobe’s response to interactive applications on the web. We could go so far as to say that it is an evolution of Flash.
Flex
And why do I say that it is the direct rival? Basically because the natural evolution of a developer with Flash will be easier to adapt to this new environment based on the same philosophy, we will see if he is able to withstand the pull of SilverLight that Visual Studio provides and the free support of MoonLight. There is also a hypothetical one JavaFX, about which you hear about but nothing is seen ;)
You can download the version for GNU/Linux from here, and the version for Win32 that is already in Beta 3 from here