'label: ^ unsub'
Google has long used smart scanning technology to find Unsubscribe links from newsletters or mailing-lists. If you search for these '^ unsub' labels, you will see all messages and use keyboard shortcuts to unsubscribe or delete multiple messages automatically.
'filename: jpg' (or replace with other formats like jpeg, gif, png)
Changing the file extension can help you find many other file types like Word or PDF. This query is useful when you need to find images.
'has: youtube'
It is possible that people often do not send video files via email because it is a bit heavy but you will often get YouTube links. This query will result in all YouTube videos. You can add a sender query or a specific time to filter, like 'from: friendo@email.com' or 'before: 12/12/2016'.
'in: chats'
By default Google saves Hangouts chats to Gmail. This query helps display all in chronological order. Adding a few keywords with this template also helps to find specific chat content. For example, if you need to find an address or phone number when chatting, type 'address in: chats'.
See also: How do other people access your Gmail without letting them know the password?