How can I configure drupal to share modules between two installations on the same server? Say I want to run siteA.com and siteB.com from the same remote server. How can I share the modules but not site specific configuration. I don't want them to have the same titles use the same modules but it seems a waste of space to be installing the same modules if they are running on the same server. 

Please note: I selected the answer below because it looked like an answer. However, seems to be useful only if the server is local? I do not use the terminal to connect to my site. I wouldn't be sure how to go about that either other than trying to do it via some shh connection.
There is a great way of accomplishing this, it's called multi-site.
See http://drupal.org/documentation/install/multi-site
and http://drupal.org/node/43816 for a start.

A great project that offers multi-site hosting is Aegir. In Aegir you can even create multiple platforms that run 1 or more sites.
See http://aegirproject.org

1 thing: you can not run multiple sites from a subdirectory if you run with Aegir/Drupal, that won't work with how Drupal is designed, but if your running on some GNU/Linux, then you might be able to symlink something together.

It's true you can use the sites/all/* folders to share modules/themes/libraries, but 1 thing is missing:
In the sites/ folder, you have to create a directory, containing the config files, files directory (etc) for every individual site that you want to run.

You could also offer multiple sites from 1 directory via an alias, but that's not what your looking for now i believe, just as an extra bonus tip. ;) Hope this helps a bit.