What are current (viable) alternatives to Reddit?

There are exactly two viable, open alternatives: Mastodon and Matrix. Neither are complete equivalents of Reddit, but both have more than 10 million users, which is what makes them viable for building communities.

If you intend to try Lemmy (an actual Reddit clone), you should be aware that Lemmy has around 9,000 users across all instances, some of which are probably duplicates, or test accounts created by developers. If you expect vibrant niche communities like on Reddit, you will likely be disappointed. This isn't a stab at Lemmy, which I believe has potential, but as of today that potential clearly hasn't been realized quite yet.

the various lemmy instances

https://github.com/maltfield/awesome-lemmy-instances

none at the momment, unless you consider "less Internet" an alternative.

then again, it took reddit about five years to become "Reddit"

4chan

Metafilter