Try Flex – online!
Our summer intern, Sara, just told me about an online service that allows you to get your feet wet with Flex without having to download/install anything. At http://try.flex.org/ you get a form with a textarea that allows you to enter some MXML-code (or select some predefined samples). When you hit the “Try It” button, the service will compile your MXML-code (I suspect they use “mxmlc” from the free Flex SDK to do the compilation) and will display the resulting SWF-file (your application) beneath the textarea. Neat!