The module is currently available for Drupal 6 and 7.
Installation and Configuration
You can request an API key from HubSpot through their website.
If Webform submissions don't seem to be working, check the Drupal error log ("Recent log entries" under Reports) and filter for entries of type "hubspot." Every attempt to insert a lead in HubSpot is recorded there, including full error messages if the attempt failed.
Alternately, enable the debugging system in the module's configuration page. Any HubSpot errors will then be emailed to you directly so you can diagnose the issue.
If you need to uninstall the module, disable it and uninstall it completely through the Uninstall tab in the Drupal module list. When the module is disabled, the Webform module won't know how to handle the HubSpot POST URL field and may throw some errors; when you uninstall this module completely, it will automatically remove the POST URL field to prevent any issues. Alternately, you can manually remove the POST URL field from each affected Webform.