What good libraries are there, in any common language, for converting PDF to HTML?
In linux install pdftohtml - For batch convertion of all files in a folder use:
ls *.pdf | xargs -I{} pdftohtml {}
This will create html site with all references and images from original documents. Every page in a separate html file. Very useful to convert project documentation to search for files by phrase, using common system file search.
Given the vagueness of the original question I'm going to go ahead and give a solution that will work with any language that can execute command-line apps. Although it can be a little bit tricky to get setup, OpenOffice can be run in headless mode on a server and, with the help of jodconverter, can convert any file format to any other file format (well, any format conversions that openoffice can handle, that is).
Here are a couple of links that help with the setup:
if you're looking for a way to convert PDF to HTML once or twice then I recommend Adobe Online Conversion
If it's an API you're after then http://www.pdfonline.com/ has an SDK that should suit your needs.
If it's a library you're after then please let us know which server-side language you prefer.