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.
4. Google Tag Manager Extension by MagePal
Allows to integrate Google Tag Manager into Magneto 2 store.
5. Twitter & Facebook Login Extension by Plumrocket
With this extension, customers can register or enter to their accounts using Facebook or Twitter profiles.
6. Shipping MatrixRate Extension by WebShopApps
Allows to set up multiple shipping options based on customer locations, so you can define different shipping rates according to a destination, weight, price or quantity of an item.
7. Chrome Toolbar (Dev Tool) Extension by MageSpecialist
If you are a developer you MUST HAVE this extension. We will not even describe it. Check! :-) And if you have a developer team, please ask them to check this helpful extension.
8. ReCaptcha Extension by MageSpecialist
Protects from spamming and brute-force. Allows to enable Google reCaptcha for customer registration, login, forgot password forms, contact us form, admin login, and forgot password forms.
9. Print Order Pdf Extension by Fooman
Advanced Order Pdf Print in the admin panel.
10. Coming someday...