The question is unclear to me. You mean phone apps to access it? Other server projects with the same idea and they're Free Software?
The notion that Reddit is an app to people hurts my aging brain.
piefed
The switch has been made
tildes.net is pretty good. i augment my use of lemmy and reddit with it.
And there are a ton of them listed here if you want to spend more time doing market research (and share your conclusions if you find anything interesting).
Amethyst
mbin
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
This community's icon was made by Aaron Schneider, under the CC-BY-NC-SA 4.0 license.