"[...]At the moment, Adobe Flash is the leading challenger. Although Flash is closely identified with showing YouTube videos and annoyingly obtrusive online advertisements, it now has its own powerful scripting language, ActionScript, and its own RIA development system, Flex. There's also an open alternative to proprietary Flash - OpenLaszlo (openlaszlo.org), which has been used by Yahoo! and, for example, the Pandora online music service."