Icons tend to be very small and it's unwise not to use the space given to an icon entirely.
It does not matter whether you are making an icon from a photograph, a 3D model or from a hi-resolution artwork. Odds are, there are unused regions near the picture edges, which do not need to be a part of the final icon.
RealWorld Icon Editor addresses this problem with two features:
Open an image and run the Image->Remove empty border command from the main menu or from toolbar.
When size is set to 100%, borders will be removed.
If the size parameter is less than 100%, borders may be added to make the object cover the selected percentage of the picture. If you are designing an overlay icon (or an icon constructed from individual parts), using less than 100% value allows you to control the size and position of objects in the final picture.
Shadows play an important role in icon design and using shadows consistently (same size and intensity) on multiple icons makes them look professional when used together.
RealWorld Icon Editor offers a standalone drop shadow filter and it is also able to add shadow right before an image is converted to icon. The application can automatically extend the canvas size to make space for the shadow.
Almost every raster editor allows you to add shadow to an image. The problem is that a 16x16 pixels picture requires a relatively different shadow than a 256x256 picture. Also, if your source image sizes are not exactly the same (and it is hard to design them the same if you do not want the unnecessary borders), which makes the same shadow in the source image look different on the icon.
RealWorld Icon Editor makes your shadow effects consistent. It lets you select the shadow size in pixels after resizing, compensates for the resizing factor and applies the shadow in high resolution for maximum quality.