Right-click on the same icon gets me a menu of other apps in that category. For example, my Firefox button of course opens Firefox when I click it. A right-click pops up a menu of the category "internet" that includes IE, Filezilla, Opera, and may also include Instant Message and IRC apps, Upload links, etc.
That sounds like a GREAT feature! I used something similar when I customized my Rhino 3D interface. It is SO convienient to have ONE button that does the most common task when left-clicked, but gives access to similar tasks when right clicked!
I know this isn't exactly what you're looking for, but I actually use Windows Toolbars. I used to hate the "Quick Launch Bar" but, with some tweeking, I hate to be without it! Pretty much what I've done, is created folders that contain the shortcuts that I use, then created custom Toolbars that display the contents of those folders. The cool thing is, you can drag the toolbars off the Task Bar onto the Desktop, then drag them off the Desktop to create a new docked window on the side/top of the screen.
As you can see: I have a new bar on the left side, the top is a toolbar pointing to my "!Shortcuts" directory, it has pretty much all the shortcuts that I ever use, all in one place. To access the shortcuts I have to press the little "
»" to open the submenu.
Below that I have a toolbar pointing to my "!Shortcuts\Common" folder, in there I keep copies of shortcuts that I use all the time, or shortcuts to programs that I don't want to forget about.
Then on the bottom I have a toolbar that displays the contents of the Desktop. I download everything to the desktop first, then decide if I want to keep it or delete it. With the handy left toolbar I can open the newly downloaded program/file as soon as it's done downloading. I can also grab newly downloaded documents and drag them onto open programs without having to minimize any windows! It works very well!
A feature I would request of a dockable toolbar: a pause before it opens (even if it's 200 ms). The one bad aspect of the left toolbar is that it opens instantly, so it'll open sometimes when I'm trying to select something on the left of the screen (be it a button, or just trying to highlight text...).
Another feature that might be cool, is to make it possible to only display files created/modified within a settable amount of time. With this feature you could have an easily accessable list of newly downloaded/created files, without having to sort through all your old files...
Also, it seems like a lot of people have shortcuts on their toolbar "just as a reminder." Having a dedicated "reminder" feature of some kind might be helpful.
And on a side note, this is the way I have firefox layed out (with the back, forward, refresh, and stop buttons, the address bar, and the google search box in the same line as the File, Edit, View, etc menus). It saves a decient amount of space...
