Flash Professional CS5 had some welcome additions for those using it to write and edit their source code, but for most developers the feature set it provides is still too limited.
There are a few options for those looking for an external source code editor. For a start there’s Adobe’s solution – Flash Builder – which has been going from strength to strength. If you’ve got the cash then FDT 4 seems to be the best option although I’ve personally not tried it. And if you already have Visual Studio on your machine then you could consider the Amethyst plug-in, which not only let’s you edit your ActionScript but allows you to debug your SWFs using Visual Studio’s excellent debugger.
But if you can’t convince your organisation to get licenses for any of the above then there’s always the excellent, and free, Flash Develop. It certainly doesn’t provide everything the IDEs listed above have, but it has some nice features, which Michael James Williams has kindly written about in his ActiveTuts+ tutorial. So if you’re still using the Flash IDE for code then I urge you to check out Michael’s tutorial and see what you’re missing out on.