Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
Zendo - Zendo allows you to sell productized, subscription and custom services. Collaborate and collect payments. Create quotes and invoices. Eliminate multiple tools and enjoy the simplicity.
osTicket - osTicket is a widely-used open source support ticket system.
Nextcloud Hub - On-premises, enterprise collaboration solution for documents, emails, calendars and more.
GreenOrbit - Intranet software with everything you need, built in
Usedesk - Usedesk is a helpdesk platform that helps automate your customer service, keep your business in sync, and your clients happy - all in one single window.