Interestingly, while the Flash IDE is most definitely closed, the SWF file format and the tools to create SWFs... it's not so cut and dry. Here's the (Flash 10 SWF Spec)[http://www.adobe.com/content/dam/Adobe/en/devnet/swf/pdf/swf...]
There's probably a bunch of stuff left out that'd make implementation difficult, but that hasn't stopped:
(GORDON)[https://github.com/tobeytailor/gordon] -- an open source Flash runtime written in pure JavaScript . It only supports up to Flash v. 2; almost useless, but interesting.
Here's the (Flex SDK)[http://opensource.adobe.com/wiki/display/flexsdk/Flex+SDK] which includes the Flex framework, and a partially open source SWF compiler.
Apparently, Flex compiler isn't totally open source because of patents concerning audio and video codecs (I'm recalling this off the top of my head; please verify).
Here's (Tamarin)[http://www.mozilla.org/projects/tamarin/]. It's a VM for EcmaScript. It's been rolled into SpiderMonkey, the JS engine for FireFox, I think. It includes support for packages, namespaces, classes, and optional strict typing of variables. Really useful stuff for interactive devs that, unfortunately, hasn't been widely adopted.
Adobe contributes to WebKit on behalf of Flash & Air; I don't know what their contributions have been/how useful they are, though.
I'm a professional interactive dev, and I specialize in Flash, so you might conclude that I'd be biased, and I wouldn't blame you at all. However, as a Flash dev, I'm familiar with the non-marketed vectors of the Flash environment. Take that as you will.
We're geeks: let's all invest a little time to investigate the rhetoric. If you really don't like Flash, pick another reason: there are many ;)