NZBHydra is a meta search for NZB indexers. It provides easy access to a number of raw and newznab based indexers. You can search all your indexers from one place and use it as indexer source for tools like Sonarr.
Not a 404, but almost
The docs for this application are not properly written yet. If you need them, please let us know in the Discord and we'll take the time to make them for you.