<p>How do I detect what browser (IE, Firefox, Opera) the user is accessing my site with?  Examples in Javascript, PHP, ASP, Python, JSP, and any others you can think of would be helpful.  Is there a language agnostic way to get this information?</p>

If it's for handling the request, look at the
User-Agent
header on the incoming request.
UPDATE: If it's for reporting, configure your web server to log the User-Agent in the access logs, then run a log analysis tool, e.g.,
AWStats
.
UPDATE 2: FYI, it's
usually
(not always, usually) a bad idea to change the way you're handling a request based on the User-Agent.