If you are looking for FREE extensions for Magento 2 here are a few of them. It is not a TOP series and extensions ordered randomly. It's just a cool stuff that we use daily and want to share with you.
1. Blog Extension by Magefan
Allows managing a professional blog on Magento 2 stores. It has the biggest number of features on the market.
2. Login As Customer Extension by Magefan
With this extension, you are able to enter customer's account via admin panel, so you can check customer issue or place an order for a customer.
3. SMTP Extension by MagePal
Allows to setup SMTP (Simple Mail Transfer Protocol) and sending emails over SMTP server like Gmail, Amazon SES, Office360, Mailgun, SendGrid, Mandrill and more.