#PixelFed is a free open federated alternative to Instagram.
It's part of the Fediverse, so you can follow PixelFed accounts from Mastodon, Pleroma etc.
You can sign up at many different instances, for example:
(You can find more instances at https://the-federation.info/pixelfed )
You can follow PixelFed's official account at:
mastodon.at is a microblogging site that federates with most instances on the Fediverse.