Was trying out these 2 Flash / Flex / Air debugger for a few days and i strongly recommend these 2 applications to any actionscript developers. It allow us to see what the frame rate, flash render time and also the memory consumation of the application we are building.
Alcon
For the Alcon, there a flash render time (the time flash take to render a frame). This is a very useful information as we should always try to keep it as low as possible in order to maintain a high frame rate. There also code like Debug.inspect to inspect object properties, however we are unable to change it values at run-time.
De MonsterDebugger
As for De MonsterDebugger, there no flash render time being display, however, we are able to change the properties of our array, object, movieclips, etc properties at run-time. This can be quite handy for fast tweaking.
For those out there who are paying more attention and focus to optimisation and streamlining of flash / flex / air, you will need to check out these 2 applications.
thinklunatic's Blog 
I would like to also point out Thunderbolt:
http://code.google.com/p/flash-thunderbolt/
This uses Firebug in Firefox and also has an AIR application available.
By: RB on November 6, 2009
at 8:56 pm
how about http://lab.kapit.fr/display/kapinspect/Integration+Guide
good for flex 3
By: framejockey on November 25, 2009
at 10:59 am
I used De MonsterDebugger.but I found it doesn’t work when I try to debug two project(swf) in the same time.It only trace info of the first open file.
Would you have any idea with this?THX
By: wolf2999 on December 14, 2009
at 9:25 am