How can I convert SVG images to Windows ICO or Mac ICN icons?

I'd like to convert svg icons to windows icons and mac icons. A free option would be the best.

There's a free online tool available for this purpose: http://iconverticons.com/

The gimp editor can open svg files and save them as ico files. From http://www.gimp.org/features/:

File Formats

The file format support ranges from the common likes of JPEG (JFIF), GIF, PNG, TIFF to special use formats such as the multi-resolution and multi-color-depth Windows icon files.

To expand on @Colonel Panic's original answer, which I feel is best, on a Mac you'll need to first install ImageMagick's CLI tools with:

brew install imagemagick

This provides the convert command, which can then be used to create a valid Mac ICO from an SVG. You'll need to resize the SVG to a valid ICO size for the OS. This is the command I ended up using to do this myself:

convert icon.svg -resize 256x256 icon.ico

ImageMagick is well documented in the following places:

• Convert Command Docs
• Magick Command Docs