I been working on a CPU emulator in ActionScript recently more as an exercise in how to write good code, try some methodologies and tools than I've not been using in other projects and improve my understanding of the CPU's instruction set. Someone will hopefully find it useful when I open source it.
We all know that code should be tested and that best practice involves writing unit tests and/or test driven development. Both Flash Builder and Flex Builder make it easy to write and run unit tests with FlexUnit so there's really no excuse not to. Despite that is the Flex SDK itself not having unit tests unit tests and the countless projects I've been involved in for one reason or the other that don't have unit tests (usually the excuse is we don't have time or budget to write tests - doh!), but that's another story.