Pixelfed is an Instagram-style photo network on the Fediverse. You can follow @pixelfed accounts from Mastodon and vice versa.
For example, @Iancylkowski is a Pixelfed account, but you can view and interact with it entirely in Mastodon too.
To create a Pixelfed account, find a server at:
➡️ https://pixelfed.org/servers
You can use Pixelfed through the web or apps.
The official apps aren't ready yet but there are good third party ones such as @vernissage for iPhone/iPad and @PixelDroid for Android
@feditips One nice thing about PixelFed (at least when I first made an account there) is that many of the accounts it suggests on the main instance are Mastodon accounts, which is a nice display of compatibility