A bunch of them are. The ones I know of: Jerboa, Thunder and Voyager. I am currently mainly using Voyager, its pretty nice
Edit: Removed Connect, as it is not Open-Source
I don’t think Connect is open-source. To my knowledge, it’s similar to Sync and Boost, a single developer working on the app as a passion project.
Also using voyager at the moment but it doesn’t feel quite right on Android as it was mostly based on Apollo for ios. Even with the slight android theming it’s still ios-like.
Thara true, but I really like its compactness and fluididity. Actually contributed a bit to Jerboa in the beginning, but its just a bit rougher around the edges than the newer clients that are now emerging.