I'm a huge fan of Boost for Drupal; the module generates static HTML pages for nodes and other pages on your Drupal site so Apache can serve anonymous visitors the static pages without touching PHP or Drupal, thus allowing a normal web server (especially on cheaper shared hosting) to serve thousands instead of tens of visitors per second (or worse!).
For Drupal 7, though, Boost was rewritten and substantially simplified. This was great in that it made Boost more stable, faster, and easier to configure, but it also meant that the integrated cache expiration functionality was dumbed down and didn't really exist at all for a long time. I wrote the Boost Expire module to make it easy for sites using Boost to have the static HTML cache cleared when someone created, updated, or deleted a node or comment, among other things.