Whether its FlexBuilder or CFEclipse, Eclipse is what I use all day long. I was thinking of compiling a list of really useful Eclipse plugins from the perspective of a Coldfusion developer. Here are the ones that I use :-
CFEclipse - Ofcourse!
XmlBuddy - Allows me to manage XML files better.
Subclipse - Subversion - I use it all the time.
AntView - Recently started using after looking into Ant.
MultiClipboard - Quite useful!
Coldfusion 8 Extensions for Eclipse - I really like the Log Viewer from these extensions.
Woul be nice to know what others use as well.
I just found out this really cool way to browse a Flex app. I use Flex Builder to develop Flex apps, so this is related to Flex Builder , not sure if this works in the Eclipse plugin as well. While you are in the code view, if you mouse over the name of a function, class or object, press the CTRL key and it becomes a hyperlink to point to the actual definition of the object.
Really cool! I always found it a bit messy to browse all the files and functions inline and dont really like the functions list view.

Just read the announcement that Flex Builder 2 would be free for students and faculty.
However, not sure if this is free for students and faculties from outside US as well. Would be quite handy if it is, its a big step forward in promoting Flex.