http://idlwave.org
1.  Compiling:  No more typing ".run myfile" a million times.  C-c C-d
C-c.  Instant compilation.  Nothing more natural when you've just edited
a file.  Compile error?  The line is highlighted.  Fix my dumb syntax
mistake, run C-c C-d C-c again.  Repeat until it compiles successfully. 
This one is an *enormous* time savings.  

2. Abbreviations:   All of the abbreviations you love (like \ine -- see
http://www.strw.LeidenUniv.nl/~dominik/Tools/idlwave/idlwave.html#TOC14
if you don't know what I mean) are available in the shell.  You can
easily add your own too.

3.  Routine info, and instant online help:   The same help features you
love from IDLWAVE buffers are available in the shell, as is the routine
info.  No more fishing through manuals to recall the ordering of those
arguments.  Instant, and I mean instant, access is available whether
you're writing a program or running one.  example:

IDL> a=histogram([C-c ?]  and you get:

Usage:    Result = HISTOGRAM( Array)
Keywords: BINSIZE INPUT MAX MIN NAN OMAX OMIN REVERSE_INDICES
Source:   Builtin 


The Keywords are all blue, so I can right click on them (the color lets
me know they have a corresponding topic in the help file).  Suppose I
right-click on "REVERSE_INDICES"... up pops the help buffer with:

      REVERSE_INDICES
      Set this keyword to a named variable in which
      the list of reverse indices is returned.
      This list is returned as a longword vector .....

all queued up.  You can just as easily see routine information and view
the source of any IDL procedure, builtin, shipped with IDL, written by
you or others... anything.

4.  Completion:  Hit tab in the shell and a routine name, system
variable, field, filename, etc. is completed.  It's even smart enough to
complete class names after 

IDL> a=obj_new('[Tab]

and keywords to that classes' Init function after 

IDL> a=obj_new('myClass',[Tab]

Those of you who use a shell like bash or tcsh that does completion
usually find that you hit Tab instinctively about every 10 keystrokes,
and the same is true in IDLWAVE's shell.  This is a *major* time saver. 
Examples.

IDL> a=readf[Tab]

and instantly I get

IDL> a=readfits(

A quick check to [C-c ?] tells me the file name is expected first, so I
begin typing it:

IDL> a=readfits('~/myfiles/[Tab]

now all my files are popped into a completion buffer (exactly the same
as if you were loading the file into emacs).  I can choose one, or get
better completion by typing a few more characters, etc.  

5.  Compiling regions:  Ever wanted to test out a bit of code but didn't
because it couldn't fit on one IDL command line (like an entire if then
begin endif else ... block)?  You can simply compile and run *regions*
if you use the shell.  Just highlight it and [C-c C-d C-e].  

6.  Debugging.  I admit it.  I too used to be a "when in doubt sprinkle
print and help statements liberally" debugger.  No longer (ok, usually
no longer).  Setting, deleting, viewing, and modifying breaks is so easy
with IDLWAVE and the shell that I just don't have an excuse anymore. 
And being able to Shift-middle click on any variable name to have it's
value printed saves time by the bushel.  Not to mention quickly
navigating the calling stack upon break or error (I suppose I already
mentioned that in a prior post).

7.  History.  The comint mode IDLWAVE uses for its shell saves the
entire history (doesn't use IDL's history).  This means the history
survives restarting IDL (an unfortunately common occurence).  And
powerful history searching (among many other features) is built right
into comint.  

8.  Shadow listing with likeliness ranking:  a fancy way of saying
IDLWAVE detects those eminently bothersome times when more than one
routine of the same name is on your path or compiled in (or builtin). 
You see this immediately with routine_info, and can even generate a full
shadow listing for everything idlwave knows about (which is a fair bit
more than IDL knows about). This works even without the shell running,
but if it is, it checks *compiled* routines also!  Especially nice when
someone dumps a big package on you to test out and you just can't seem
to get it running, or can't run your old stuff alongside it.

I could go on and on but I won't.  Emacs and all this lisp business is
intimidating at first, but you really don't need to know any of that to
get all the benefits.  Customization usually just involves cut and paste
to your .emacs file.  Don't let the parentheses scare you.  Give it a
try, and you'll see why I spend so much time raving about it.

Thanks Carsten!

Good luck,

JD

P.S. And for those of you afraid you won't be able to install idlwave...
it is simple.  All you do is download:

idlwave.tar.gz
idlwave-help.tar.gz

to /tmp (or anywhere), then do (as root):

% tar xzvf idlwave.tar.gz
% cd idlwave-version
% tar xzvf ../idlwave-help.tar.gz
% make
% make install-all

where "version" is something like 4.5.  This by default puts idlwave in
/usr/local/share/emacs/site-lisp/, which is a fine place.  It also puts
the help files in /usr/local/etc, which is also fine.  (Carsten had to
separate these for copyright issues, not to punish us).

Now simply add the following to the top of your .emacs file.  (Note
possible changes):

;;;; IDLWAVE setup
; Comment the following out if emacs already knows about 
; where idlwave is installed.  Try M-x idlwave-mode to see.
; You can also change the path if you've put it somewhere strange.
(setq load-path (cons "/usr/local/share/emacs/site-lisp/" load-path))

; Change the following to point to the idlwave help files,
; if installed somewhere other than the default.
(setq idlwave-help-directory "/usr/local/etc")

; Font lock, not just for IDLWAVE... mmmm colors
; comment out if you're boring.
(global-font-lock-mode 1)
; Uncomment the following if you commented the above,
; but would like font-lock to operate in IDLWAVE mode.
;(add-hook 'idlwave-mode-hook 'turn-on-font-lock)

; Here we ensure sure IDLWAVE is loaded for .pro files...
(autoload 'idlwave-mode "idlwave" "IDLWAVE Mode" t)
(autoload 'idlwave-shell "idlw-shell" "IDLWAVE Shell" t)
(setq auto-mode-alist  
 (cons '("\\.pro\\'" . idlwave-mode) auto-mode-alist))
;;;;;;;;;;;;;;;;;;

That's it to get going.  See idlwave.html#SEC40 for more customization
fun.  (I for one insist on (setq idlwave-main-block-indent 3) -- with
due deference to David Fanning's coding style).  And don't forget to
build your personal catalog of routines (menu IDLWAVE->Routine
Info->Select Catalog Directories) for maximum enjoyment!