you could just use HTML conditional statements:

<!--[if IE]>
<link href="/includes/ie-fixes.css" rel="stylesheet" type="text/css" />
<![endif]-->

there is also get_browser() for php:

$browser = get_browser(null, true);
print_r($browser);

personally I use the html method.. i've used the php method and i like it but it won't work on all servers if it's not setup properly.