jquery.
easy peasy.
Code:
$(document).ready( function(){
if ($.browser.webkit) {
window.location = 'http://www.google.com/';
}
else if ($.browser.msie) {
window.location = 'http://www.yahoo.com/';
}
else if ($.browser.opera) {
window.location = 'http://www.opera.com/';
}
else if ($.browser.mozilla) {
window.location = 'http://www.firefox.com/';
}
})
etc ...
http://api.jquery.com/jQuery.browser/