As I've used it in two posts I should mention what wonderfl is. It's simply one of the best resources available for Flash developers, in particular ActionScript programmers. It lets developers upload and run ActionScript, for testing or to demonstrate it. Code on the site can be modified, even if you did not write it, by creating a copy (or a 'fork') and working on that. Editing and testing all takes place in the browser, making it often much quicker than setting up a similar project in Adobe Flash CS or a similar environment.
As with all programming discussing ActionScript often involves discussing code. As code is text it can be just copied into a discussion, or dumped somewhere like Pastebin. But this means other developers must read and understand the code, easier for some than others (reading other developers' code is not a required skill for Flash developers), and highly dependant on the quality of the code.
A compiled .swf can be uploaded to Newgrounds or MegaSWF with the code uploaded somewhere else. But this is extra work for both the uploader and readers, as they need to look at the code and the binary separately.
And far too often I've come across posts where someone is discussing a method or asking a question, and have provided both a running application and a code sample, still it isn't possible to understand the point they are making or the question they are asking. Often the code depends on some other code that's not included, or the behaviour demonstrated by the application is over too quickly and doesn't repeat.
Wonderfl helps deal with all of these issues. Code uploaded to wonderfl compiles as soon as you stop typing into a running application. The application is hosted alongside, for easy viewing, so the person posting the code need provide only one link. It encourages developers to post compact, working code, that doesn't depend on the timeline or architectural features of their games.
It's biggest strength is the ability to 'fork' code, i.e. to make a copy for yourself which you can then work on as your own. If you think you can fix a problem someone has posted, or think the example is too short and want to make it run longer/more often, you can fork the code and change it.
It's also very interesting to browse. There are all sorts of examples, some very large and sophisticated, including full games. Again in most of these the code can be examined and forked, so it's easy to find out more about any example. Flash 11 only, but that's understandable as a lot of Stage3D examples seem to be appearing. Altogether a great resource for Flash developers of all sorts.