Although jEdit was not built with ActionScript in mind, its flexibility and expandability through macros and plugins make it an ideal cross-platform editor for ActionScript development. It may be of particular interest to OS X and *nix users, for whom no dedicated AS editor yet exists.
jEdit is a full-featured, cross-platform editor written in http://java.sun.com and supported by a growing number of developers.
The project is hosted on sourceforge.net