What is Ecko?

Ecko is a Mastodon fork with many cool features, that Mastodon doesn't have.
It follows the Collective Code Construction Contract or C4 to e.g. add features that are deemed valuable by the community.
This is a difference from Mastodon, where Gargron decides which features Mastodon should have on his own even against the public opinion.

What features does Ecko have, that Mastodon doesn't?

The admin can change the following values in the config file:

max post length
max poll options
max poll answer length
max profile metadata fields
max length of “about me” text
max number of pinned toots

The admin can also upload a favicon in the admin settings and import/export the instance blocklist.
Furthermore local toots are possible. Those are toots that can only be seen by people on the same instance.

Where can I find an Ecko instance to join as a user?

You can try the official instance here.
There is also a full list of Ecko instances here.