MXML/AS3 editor or development environment besides Flex Builder

Aptana

An integrated development environment for creating Adobe AIRTM (formerly code-named Apollo) applications.

Intellij IDEA

IntelliJ IDEA is a commercial Java IDE by the company JetBrains.It allows programmers to quickly redesign their code.Many advanced features are supported by the IntelliJ 7.

  • Smart code completion for MXML and AS files, with cross-resolution of code symbols between them, even inside of mx:Script blocks and attribute values
  • Automatic code formatting, highlighting and styling
  • On-the-fly code validation with instant quick-fixes
  • Code insections to hunt perfomance bottlenecks, bad code practicies and other problems
  • Rich set refactorings
FDT

FDT is an ActionScript plugin for Eclipse developed by Powerflasher.It provides support for AS3 and Flex development projects with syntax highlighting, code hinting, code folding and Live Error Highlighting to name a small percentage of the overall feature list.

SEPY

SE|PY is a FREE ActionScript editor for Adobe/Macromedia Flash files.

FlashDevelop

FlashDevelop is a .NET open source script editor designed mostly for Actionscript 2/3 development.