MTASC and SWFMill (FAME) combo
A sustainable(?) command line method/process of building Actionscript Flash applications from the command-line and still use Flash movie-clip resources without using the authoring environment. Now I need a good IDE for Actionscript and hoping Eclipse works out...
UPDATED 05/17/05: here's Peter Joel's blog about setting this up in Eclipse (real-time syntax checking) and Subversion/Eclipse integration!
UPDATED 06/02/05: More Eclipse plug-ins: Flash Log and Flair SWF viewer. And a link to a "why I switched to FAME" blog.
UPDATED 07/19/05: Runtime debug tool. Looks pretty interesting. FREE!