Tip: Make Your Hidden Applications Translucent
One of my favorite features in OS X (and probably one of the least used ones for that matter) is the ability to “Hide” an application rather than just minimizing it to the dock.
However, once a window is hidden, it’s really not that useful because you can’t even tell that it’s hidden, other than not seeing it.
You can always use the little triangle under the dock icon (if you’re on Tiger) or the blue light (on Leopard), but I’ve found a quick trick that makes hidden windows a little easier to work with, and to know if they’re actually hidden. It makes use of an OS X tweaking app called MacPilot from Koingo Software. Read along to see how I did it.
Although MacPilot is a shareware app, you can still use it for free for 15 days, which is more than enough to make your desired tweaks. For this tweak, you want to head over to the “Dock” tab in the application. You’ll see that there are literally hundreds of mods and tweaks that you can make to your system from this incredibly handy application.

To make your hidden applications more distinct from your other applications, you simply check off the “Hidden applications have translucent icons” option in the second column. Click on “Apply Changes” and it will tell you that it needs to reload the Dock to make your changes visible.

Now, it’s much easier to notice the difference between hidden windows and non-hidden ones, with a simple little trick. In the Image below, Safari is hidden, while the other applications are not.

Note: If you’re comfortable in Terminal, you can run this command from the Terminal application, located in the /Applications/Utilities folder of your Mac.
defaults write com.apple.dock showhidden -bool true
killall Dock
You can also do it with a GUI application by using MacPilot. MacPilot is $19.95 from Koingo Software, and there is a 15 day free trial available, but if you find this app as useful as I do, you should consider purchasing it.

Or you can use the free TinkerTool.
…or you can just look at which icons in the dock are translucent…
(hidden icons are translucent *by default*)
@guest: I don’t know what version of OS X you’re using but on my fresh install of Leopard, none of the hidden applications showed translucency by default.
hidden applications are NOT translucent by default. but, yes, tinkertool (freeware) can enable this, as well as onyx (freeware), and the reason they both can is because os x can actually do it by default:
you can do this without any extra program in the terminal.
see, e.g.:
http://osxdaily.com/2007/05/17/make-hidden-applications-transparent-in-the-dock/
Something else in MacPilot:
Release Notes (version 2.4.5; released on: 2007-11-19)
* New Feature: !!! Make the Leopard menubar opaque (remove transparency)!
I swear I haven’t installed any software that controls this sort of behaviour. Below is an image of both Terminal and Safari hidden via cmd+h.
http://img261.imageshack.us/img261/9637/hiddenwf0.png
As far as I was aware, it’s always done this, including in Tiger. (My girlfriend’s Macbook was Tiger too – I didn’t set it up, and it does the same thing. After her upgrade to Leopard, it does as well. Same for my dad’s macbook.)
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