function formHandler(form){
var URL = document.form.site.options[document.form.site.selectedIndex].value;
window.location.href = URL;
}

document.write('<form name="form">');
document.write('&nbsp;&nbsp;&nbsp;<select name="site" size=1>');
document.write('<option>S&eacute;lectionner un article ...');
document.write('<option>À venir bient&ocirc;t !');
document.write('</select>');
document.write('&nbsp;&nbsp;<input type=button value="Allez-y !" onClick="javascript:formHandler()">');
document.write('</form>');