Catodon is a fork of Sharkey, but with many features ported from Firefish, and some new ones! Its target is to provide a smoother, more comprehensible UX - so terminology is simplified and made more descriptive, and several features of Misskey/Sharkey that were not widely used or not federated were removed from the frontend. There is also an attempt to make the design more minimal wherever possible - see the redesigned posting form for example. One of the main design goals is to make something more user-friendly, where you can invite your friends and they can adjust easily.

Please join our Matrix space at #catodon-space:matrix.org for direct communication! Feedback and code reviews are much appreciated.

Main differences from Sharkey:

A new, rounder look!
You can follow hashtags and words in you Home timeline, by adding Custom feeds (Antennas) to Home
You can integrate GIF search for your users through Klipy
Improved notifications: Added a "mark all as read" button to the Notifications widget, there is a highlighted divider between new and old notifications, and they are marked read easier when you view them, so that you no longer need to use the "mark all as read" button as much. You can also see a list of who reacted in grouped reaction notifications
Threads in timelines (imported from Firefish), greatly decluttering timelines for longer threads
Users now have the choice to link to external media (including GIFs) without permanently saving them to their Storage, using a new dedicated cache - allows admins to provide less permanent storage space per user, keeping server resources under control
There is a global, admin-definable cap to the cache for remote files, so that you no longer need to manually delete it regularly
Different/simpler terminology so that new users can understand what everything means more easily
Decluttered buttons under posts: Like/Reaction (merged), Reply, Repost/Quote (merged)
Deprecated/removed from the frontend the gamifying or local only/not widely used features: Channels, Achievements, Gallery, Clips, Games, Chat, Play
Redesigned left sidebar. Much fewer items in the Navigation Bar, which now expands/collapses with a button. The profile picture now acts as a direct link to your profile, as it's separated from the account menu that sits under it
Fewer timelines, no hardcoded links in the header menu - everything is a timeline
"Accounts timeline" based on the "Following" page, but upgraded to a timeline with columns scrolling independently and a search bar
The local timeline now takes the server's short name, adding a unique touch to your community
Custom feeds and Lists are in their own dedicated timeline
More filters in timelines and Lists - you can now view your following accounts' replies to others (or only to accounts you also follow) in Home, choose between Following and Mutual followers, and include the public posts from your server in Home (covering the functionality of the deprecated "Social" timeline)
Post area is now clickable everywhere, not only over the text area
You can view all reactions together under a post in detailed post view (added "All" tab). You can also see the total number of replies+subreplies next to the number of direct replies, showing instantly how deep the discussion under a post goes
Redesigned posting form with buttons in more practical positions - also the collapsed posting form at the top of timelines takes up half the space the old one did
The posting form autoexpands as you write longer posts - imported from Firefish
Autocomplete also shows results for names of users - you no longer need to remember the exact username to mention or DM someone
You can disable sign-in emails by deactivating sign-in notifications
You can now close the alt text window over images, which can make parts of them hard to see/read, by clicking on it
Much better key navigation - both with h/j/k/l and arrows. Focus is more often where you expect it to be, so that you can key nav instantly when opening a page. You can also swipe through timelines on desktop with left/right
Countless bug fixes and quality-of-life improvements!

Only server i found: https://catodon.rocks/