Here is a list of ten anime series that you can watch for free on official streaming platforms:

1. Naruto Shippuden: Available on Hulu with a free trial period.
2. Sword Art Online: Available on Hulu with a free trial period.
3. One Punch Man: Available on 9anime with a free trial period.
4. Attack on Titan: Available on Hulu with a free trial period.
5. Death Note: Available on Netflix.
6. Cowboy Bebop: Available on Funimation with a free version.
7. Fullmetal Alchemist: Brotherhood: Available on Netflix.
8. The Seven Deadly Sins: Available on f123movies.
9. Yu-Gi-Oh!: Available on Hulu with a free trial period.
10. My Hero Academia: Available on Funimation with a free version.

It's worth noting that the availability of anime series on streaming platforms can change, so it's always a good idea to check for the most up-to-date information. Additionally, some platforms may only offer a limited selection of episodes for free, so it's important to read the terms and conditions before watching.