Slack: Slack is a widely used team communication platform that allows real-time messaging, file sharing, and integration with various business tools. It offers channels for different teams or projects and supports both desktop and mobile devices.

Microsoft Teams: Microsoft Teams is a collaboration platform that combines chat, video meetings, file storage, and integration with other Microsoft tools like Office 365. It offers features like channels, private messaging, and customizable workspaces.

Google Chat: Google Chat is a part of the Google Workspace suite and offers messaging, file sharing, and collaboration features. It integrates seamlessly with other Google tools like Google Drive, Docs, and Calendar.

Cisco Webex Teams: Cisco Webex Teams is a business messaging app that provides secure messaging, video calls, and file sharing. It also offers integration with Cisco's conferencing and collaboration tools.

Mattermost: Mattermost is an open-source, self-hosted messaging platform designed for teams. It provides real-time messaging, file sharing, and integrations, with a strong focus on security and privacy.

Flock: Flock is a team communication and collaboration app that offers messaging, video calls, and file sharing. It also includes features like shared to-do lists, polls, and reminders.

Workplace by Facebook: Workplace by Facebook is a communication and collaboration platform designed for businesses. It offers messaging, group collaboration, and integration with various Facebook tools, making it suitable for organizations familiar with the Facebook interface.

These are just a few examples, and there are many other instant messaging apps available for businesses, each with its own set of features and strengths. The choice of the app depends on your specific business requirements, team size, and preferred integrations.

Using a chat API can be an efficient way to add instant messaging capabilities to your business applications or services. Many Free Chat API provide pre-built infrastructure and functionality, saving you the effort of building everything from scratch.