I use TextMate with the ActionScript bundle. But, that is a far cry from a decent development environment.
FlashBuilder or FDT are probably the best options. I've spent a lot of time looking for a good free option - though I haven't done a check in the last 6 months, or so. The actual Flash IDE might be the best no-additional-cost option, if you use Flash.
There is supposedly a way to set up your environment to use XCode, a stand-alone compiler, and one or two other things, but I could never find good documentation or evidence of someone actually getting things working.
There is, or was, a Mac version of SePy, but it was horrible, as of a year and a half ago when I gave up on it.
I've been using FlashBuilder on my work machine for Flex work, but I think I'll start using it on my personal machine for Flash work. Just haven't come across anything else that even approaches a modern dev environment for Flash.
Sorry I don't have more helpful information. Would love to hear someone say different.
UPDATE: Found some so-so info on setting up various elements of a full development environment on Mac, using free tools. Doesn't look like a lot has happened on this front in the last two years.
OpenCode - AS2 and AS3 language definitions for XCode. This goes back to 2006, but may still be completely useful.
Xcode and the Flex SDK - A tutorial on setting up a code and compile environment with XCode and the mxml compiler. Author notes that the information may not be 100% accurate and lost interest once he started using FlexBuilder.
Make Xcode a Full Featured Actionscript IDE - FlaXIDE - A tutorial on setting up a full dev environment with opensource tools. Last updated in 2006. Could be modified to work with current tools (i.e. haxe instead of mtasc).
Flex Support for Xcode 3 - Somewhat more recent info on using Xcode as an AS editor. Posted November of 2007.
Core SWF: Flex/AS3 for Xcode - Even more recent info. This seems to be the latest thing going for using open source or free tools to build an AS dev environment on the Mac. Posted July of 2008. This project is up on Google Code and contributors are welcomed.