Drupal, PHP, and APC
Tuning APC (Alternative PHP Cache) to work with Drupal is a relatively painless process. As Dries Buytaert has noted, APC can help serve Drupal pages up to 4 times faster than a stock Apache and PHP setup.
However, tuning APC for use with more than one Drupal instance can be a little bit more challenging than a single site running on it's own server.
