CiviCRM no longer officially supports Drupal 6.x. as of version 4.2 but seems that more than one is running versions above 4.1 with Drupal 6 site.
Up to what CiviCRM version I can upgrade my Drupal 6 site without suffering any incompatibility?
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Pete is right that even recent versions of CiviCRM (certainly in the 4.6.x series) work with Drupal 6 sites. The "no longer officially supports" issue is that the core team won't go out of their way to make D6-specific fixes.
That said, I have encountered problems with new installations on Drupal 6, but this has been for testing--why else would someone build a new site on Drupal 6?