As a Flex developer I tried several IDE’s for developing certain projects in a faster and easier way. I’ve tried some open source/free IDE’s but none of them was as good as Flex Builder because they didn’t had the Design feature for dialogs and components. The main reason is the memory and the process that Eclipse environment ( on which Flex Builder is based  ) occupies is too much.

Recently after a common google search I’ve encountered a Microsoft Visual Studio plugin that gives you the posibility to develop Flex applications and also with a design posibility like Flex Builder. Under the name of  “Amethyst” by SapphireSteel software under the current development state  of beta 5, this plugin can be downloaded from here

This seems to work a lot faster, and the good part is that it has the debugging feature from Visual Studio which is more friendly that the one from Flex Builder or so it seems to me.

Click here for details on installing and using this plugin.

Here you have some screenshots on this

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Good Luck.