Code:
<script type="text/javascript">
function bookmarksite(title,url){
if (window.sidebar) // firefox
    window.sidebar.addPanel(title, url, "");
else if(window.opera && window.print){ // opera
    var elem = document.createElement('a');
    elem.setAttribute('href',url);
    elem.setAttribute('title',title);
    elem.setAttribute('rel','sidebar');
    elem.click();
} 
else if(document.all)// ie
    window.external.AddFavorite(url, title);
}
</script>


<a href="javascript:bookmarksite('Mike\'s Place', 'http://www.google.com')">Bookmark this site!</a>