My development environment varies based on the project that I'm working with.  For my latest project I'm creating a Flex media player and though I started using Gedit, I've moved to Eclipse + Flex Builder so as to take advantage of Flex's Builders code completion feature.  Other features that I needed for this project were support for FTP, Bazaar, PHP, and JQuery.  Here are the steps I took to build the IDE for this project.