Boost Module for Drupal Caching
Boost is recommended to help with caching in Drupal 7 for "anonymous" traffic. That's users that aren't logged in. I installed it here at PcHousekeeping a few days ago and it really seems to have picked up the speed.
The Boost project page says that it's "an advanced module and takes some extra effort to get it working correctly", but to me it was no harder than most. It will work on shared hosting which is a plus for lot's of folks. Drupal.org has a good list of resources entitled Drupal caching, speed and performance if you really want to dig into it all.