There are different third party development tools availabel for flex. They are following.
1. Flex Builder: THis is my favorate IDE for flex application development. I am using it every day. Its based on Eclipse and as I have a strong Java background it is natural platform for me.
2. FleshDevelop: It is really very powerful IDE that supports flex as well as Flash, AS2, AS3 and AIR development. It is very fast. The only one drawback is it doesn't support other plateform then windows.
3. FDT: It comes to serious ActionScript development, especially with its code completion, documentation support and very much missing feature in Flash Builder code formatting.
4. IntelliJ IDEA 8.0: In Java word IntelliJ is seen by many to be the best of all IDE?s so I expect their Flex plugin can become very important player in our environment.
6. Tofino: It is a free microsoft visual studio plugin. It is still under active development but there are some builds already available to download and start coding. I?m hoping to see with this one more and more .NET developers attracted by our great Flash Platform. With great .NET server side capabilities and real cross platform UI with Flex I believe it will rock!.
7. Amethyst Personal Edition: It is another Visual Studio plugin.
8. FlexBean: It is a NetBeans plugin.
9. AXDT: It is a set of plug-ins for the Eclipse plateform that enable you to write ActionScript3 code in an integrated environment. AXDT is based on Eclipse Imp and utilizes the opensource FlexSDK.