The sysadmins here were doing some software upgrades the other day and asked me which versions of Tomcat and Apache Solr they should install on our shared Drupal hosting servers. The answer is a little complex, so I finally just drew them a diagram.
Note that in Drupal 7 there are two completely unrelated modules which can talk to Apache Solr: Apache Solr Search Integration, which has existed since Drupal 6 and is primarily developed by Acquia, and Search API Solr search, which uses Drupal 7's Search API family of modules. Both have their advantages and disadvantages, but overall the Search API approach provides more flexibility because it provides a standardized way for module authors to customize the results from several different search backends. With Search API you can also switch search engines with relative ease, should you ever choose to do so.
Here's the chart (click for a larger, more readable version):
