There are a couple of ways that you can view your unsubscribed contacts:

Viewing unsubscribes by campaign

To see contacts that have unsubscribed after receiving a particular campaign, start by navigating to your Sends or Campaigns menu.
From here:

• Click the more icon next to a running or finished campaign, and then select View Report.

• Once you're within the campaign report, click on the Metrics tab (1).
• Scroll down to the Unsubscribes section (2) where you will see details for all contacts that have unsubscribed through this campaign.
• You can also download this list using the Download link (3). *

Note: You can also view Bounced emails (4) per campaign the same way.

To see all unsubscribed contacts

Start by navigating to your Contacts menu.
Within the Overview tab (5), you will see the Unsubscribed Contacts section (6).

Click View All at the bottom of the section to see details for all your unsubscribed contacts, including the date and reason they were unsubscribed.  These reasons may include by method of Contact, Integration/CRM, or manual.

The Other section refers to those contacts that cannot be reached by reason of Soft/Hard Bounce, Blocked, Failed Validation/Bad Email or Duplicates.

* Download functionality is not available to all users, and can be disabled upon request.