I suggest you do this on your static pages before rendering to the browser, or you'll be pushing the burden of conversion computation onto your poor visitors. :) Here's how you might do it in Ruby (reading from stdin, writing to stdout):
while line = gets
puts line.gsub( /(^|[^"'])(http\S+)/, "\\1<a href='\\2'>\\2</a>" )
end
Obviously, you'll want to think about how to make this as robust as you desire. The above requires all URLs to start with http, and will check not to convert URLs that are in quotes (i.e. which may already be inside an <a href="...">). It will not catch ftp://, mailto:. It will happily convert material in places like <script> bodies, which you may not want to happen.
The most satisfactory solution is really to do the conversion by hand with your editor so you can eyeball and approve all substitutions. A good editor will let you do regexp substitution with group references (aka back references), so it shouldn't be a big deal.