So far, there are four major technologies for the RIA.
1. Flash | Flex from Adobe
3. Silverlight from Microsoft
4. JavaFX from Sun
Flash is around for many years and clearly the most powerful one, specifically with the Flex, developer friendly RIA development environment build on top of Eclipse platform.
Ajax is very strong and popular and many sites use the technology. On the other side, Microsoft entered the arena with their Silverlight technology. Supported in Windows and Mac only at this point, however, Linux corresponding Moonlight project allows to run Silverlight apps.
And finally, Sun's JavaFX showed up last year, still in development, but very promising.
Let's see what will happen in the near future...