Horizons Unlimited was initially a combination of multiple systems (Drupal 5, static pages, vBulletin, movable type) that was developed in-house over several years. My work included upgrading Drupal 5 to Drupal 6, upgrading vBulletin to the latest version, setting up custom theme integration between Drupal & vBulletin (partially documented in the issue queue for DrupalVB), fixing session & cookie issues between Drupal & vBulletin, merging external systems' functionality into Drupal, and eventually migrating all content & users (from both Drupal & vBulletin at the live site) into the upgraded version.