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Yep, you can. Right-click on the App in the Application folder. Click on "Show Packages Contents." Look in the Resources folder for the app icon, it has a .icn extension.

You will need to make an icon of your own with which to replace the stock icon. I'm not sure what the dimensions are, but I'm willing to bet you can find it online. Or, open it up with Photoshop or Gimp.

Be sure to save your new icon in the App's package with the proper extension.
 
In Finder find the App that you want to change the icon, right click and select "Get Info." Do the same thing with the icon you want to use. On the top left of the Get Info window click on the small image of the icon so that it's highlighted in blue and copy (Command + C). On the Get Info box of the application, highlight the icon on the top left so that it's highlighted in blue and past (Command + V).

This should work for most of the dock icons. For me it would not work on Finder, Trash or iCal.
 
In Finder find the App that you want to change the icon, right click and select "Get Info." Do the same thing with the icon you want to use. On the top left of the Get Info window click on the small image of the icon so that it's highlighted in blue and copy (Command + C). On the Get Info box of the application, highlight the icon on the top left so that it's highlighted in blue and past (Command + V).

This should work for most of the dock icons. For me it would not work on Finder, Trash or iCal.

For simple and safe changes to the Finder, Dashboard and Trashcan icons (as well as virtually every other system icon including alias badges, folders, etc.) you can use the free utility LiteIcon which can be found here.

In action it looks and acts much like the commercial CandyBar- but will not allow you to change Applications icons so simply like CandyBar will.

The application iCal, because of the current date being displayed as a dynamic part of the dock icon, is in another class regarding icons changes.
 
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