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);
}
<a href="javascript
:bookmarksite('this site', 'http://www.thissite.com')">Add this site to your favourites</a>