Microsoft Outlook is a popular email service that is used by millions of people worldwide. It comes with a range of features that can help you manage your emails, calendar, and tasks. However, with the right add-ins, you can take your productivity to the next level. There are many must-have apps for Outlook that can make your life much easier. Some of these extensions are paid, but most of them are free to use. Here are some of the best add-ins for Outlook:

1. Custom Signatures: This add-in allows you to create and manage custom email signatures. With this app, you can add images, links, and social media icons to your signatures, making your emails more professional and informative.

2. Asana for Outlook: Asana is a popular project management tool that can be integrated with Outlook. With this add-in, you can create tasks, assign them to team members, and track their progress, all from within Outlook.

3. Attachments: This app makes it easy to manage email attachments. With Attachments, you can save attachments to your cloud storage accounts, compress large files, and even convert files to PDF format.

4. Calendly for Outlook: Calendly is a scheduling app that can be integrated with Outlook. With this add-in, you can share your availability with clients and colleagues, and allow them to schedule meetings and appointments directly from their email.

5. Invoice and Time Tracking - Zoho Invoice: This app allows you to create and send invoices, track time spent on projects, and manage expenses, all from within Outlook. With Zoho Invoice, you can streamline your billing and accounting processes.

6. TimeZone Finder: This add-in makes it easy to schedule meetings with people in different time zones. With TimeZone Finder, you can quickly find out what time it is in different parts of the world, and avoid scheduling conflicts.

7. SignOnTheGo: This app allows you to sign and send documents directly from Outlook. With SignOnTheGo, you can sign documents electronically, and avoid the hassle of printing, signing, and scanning documents.

8. Writer for Outlook: This add-in provides you with a distraction-free writing environment within Outlook. With Writer, you can focus on writing your emails, without the distractions of other apps and notifications.

9. Cascade for Outlook: This app allows you to create and manage email templates. With Cascade, you can save time by creating templates for common emails, and quickly customize them for each recipient.

By using these must-have apps for Outlook, you can streamline your workflow, save time, and increase your productivity. These add-ins can help you manage your emails, schedule appointments, track time, and even sign documents electronically. Whether you're a small business owner or a busy professional, these add-ins can make your life much easier.

Here is a simple step-by-step guide on how to install add-ins in Outlook

Note: If you want to disable or remove an add-in, you can follow a similar process by going to the "Add-ins" section in the "Options" menu and selecting "COM Add-ins" to manage them.