February 2012 Edit for an old journal entry:

Way1: Photoshop Pluggin

1-Download: [link]
2-Cut
3-Paste : C: \Program Files\Adobe\Adobe Photoshop CS3\Plug-Ins\File Formats

When you want save "Png" file, choose ".ico" file format in photoshop.
You can save with 256 X 256 px. But icon file will have pixel problem if you need/use with smaller size than 256 or 128px.

Way2: Better one!
IcoFX is nice application for change file format.
It's successful for .ico and .icns and you can use for batch works.

Way3: You can use Gimp.

Way4: convertICO.com

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Thanks to:
for telling different ways except than file format pluggin.