What kind of server do you have. Personally I would suggest PHP.

Like you mentioned it is free, 60% of the worlds webservers are running apache, apache and php go hand in hand. Also there are copious amounts of free information on how to do different things with php. www.devshed.com, www.phpbuilder.com Also they have great documentation at www.php.net

The debate about PHP vs. ASP can be very heated so I will refrain from talking down about an M$ product