I highly recommend this debugger. It is an AIR application that has many useful features and will make your life much easier. Some of those features are:
- detailed traces (trace strings and objects, which you can expand)
- edit properties at runtime
- method testing
- a tree structure with which you can walk through the application
On top of that, it is open source and free!
You can download De Monsterdebugger here. I had a problem with the download though. The .zip file is actually a .air file. So you might have to change the extension in order to install it.