Overview
The Authcache module for Drupal offers page caching for both anonymous users and logged-in authenticated users. This allows PHP to only spend 1-2 milliseconds serving pages. (Click "Authcache Debug" on the top left for page stats.)
Download
http://drupal.org/project/authcache
How to Test this Module
Create an account (no email verification required) and click back and forth between pages. The first hit may not be cached (since the server must first save to the page to its cache); any future hits will be. On new comments, node updates, etc, the cache is cleared/invalidated.
How does it work?
Authcache gzips & saves the final rendered HTML of a page to serve visitors. A separate cache is created for each user role as defined by the administrator, so some roles can be excluded if necessary.
Authcache places priority on serving pages to the visitor as fast as possible. After a page is loaded by the browser, a second HTTP request may be performed via Ajax. This initiates a lightweight Drupal SESSION bootstrap which allows SQL queries to be executed (such as updating the user "history" table or node "statistics" table), and returns any user-customized data to the page (such as form tokens or default values on a contact form).
