Yesterday Adobe released their Flex SDK under an open source license. The license they chose was the Mozilla Public License (MPL), which is a fairly common license but is not GPL compatible. The license is more corporate-freindly than GPL, which is probably the reason for this choice. As the Flex SDK has been free since Flex 2 was released, this may not seem like that big of deal. What it does do is confirm that the Flex SDK will always remain free.
For those of you who are not familiar with what Flex is, it is a pure code-oriented way of creating Flash content. Flex projects consist of MXML files which is an XML-based layout language. Putting together a flex application is done in a similar way to creating a web page. That being said, flex supports ActionScript 3, so you can also use the SDK to compile ActionScript files. All of this code can be created in a text editor, though Adobe has a commercial tool called FlexBuilder that gives you a graphical way of putting together Flex applications.
Now that I own a copy of Flash CS3, I really don't need to use the SDK to create ActionScript 3 programs anymore, but I do still plan on keeping my code in separate files so that when I do Open Source my projects, people will be able to use the free Flex SDK to build and modify my code. Having the SDK free gives me added security as I know that if Microsoft does somehow manage to kill Adobe, Flash could still survive without them.
The Flash 9 player is still closed source. As the output of the Flex SDK is a file that requires the Flash player, some people may argue that open sourcing the Flex SDK is nothing more than a gesture. While I personally would like the Flash player to be released as open source as well, I know from my Java experiences that having multiple vendors creating versions of a platform leads to incompatibility problems so I personally don't mind that there is only one official source for the Flash player. That being said, there are open source alternatives to the Flash player, with the biggest one being Gnash which is aiming to be compatible with Flash 7, though I suspect they will eventually support Flash 9.