IMO this is one of the biggest blunders of HubSpot. Tagging is one of the most important features of any CRM, to assist in searching, categorizing, etc. This solution that people suggest of "adding custom fields/properties" is a workaround that is cumbersome at best and actually doesn't really address the missing functionality. HubTags seemed like an ideal solution, but they just went under. Hubspot really needs to add this as a feature, ASAP!