This code allows your visitors to add your site to their favorites with a link on your website.

With Text Link:
<a href=":Add This Page To Your Favorites!" onDragstart="return false" onClick='window.external.AddFavorite(location.href, document.title);return false'>Add This Site To Your Favorites</a>

With Image Link:
<a href=":Add This Page To Your Favorites!" onDragstart="return false" onClick='window.external.AddFavorite(location.href, document.title);return false'><img src="http://YOUR IMAGE URL GOES HERE" border="0" alt="Add this site to your favorites!"></a>