We use JIRA because it's a single tool that can be customized to each of our business processes: product development, customer on-boarding, and internal operations JIRA
We use Confluence for specifications and knowledge sharing. We chose Confluence over other wikis primarily because of its integration with JIRA Confluence
We use Zendesk for customer support and hosting documentation. Additionally, we provide documentation search from within our application using Zendesk's API Zendesk