The Zendesk Marketplace lists apps for the agent sidebar (showing CRM or order data next to a ticket), full two-way integrations (syncing tickets with another system's records), and standalone utilities. There are listings for connecting with Salesforce, NetSuite, ServiceNow, Asana, Microsoft Teams, Airtable, and Slack, among many others, built either by Zendesk itself or by third-party partners.
Install one, authorize it, and in most cases you're looking at relevant data from the other system right inside a ticket within minutes. For common workflows (showing a Salesforce account record, pushing a new Zendesk customer into a CRM, posting ticket updates to a Slack channel) a marketplace app is usually the fastest path and the one worth trying first.
If you want something more custom but still want it to live inside Zendesk's own interface, the Zendesk Apps Framework (ZAF) lets you build a private app for just your account, with its own panel in the agent sidebar, without needing to publish it publicly.