I use Mast, created by @JPEG, to browse the Fediverse on iOS. It's a pretty great client, and open-source. However, it was somewhat recently sold, and I couldn't find the code – that is, until today. Check it out: https://github.com/tiagomartinho/Mast2
@JPEG is now working on a Twitter client, Aviary. I've downloaded it, and so far it's pretty great. Nice job.
Fosstodon is an English speaking Mastodon instance that is open to anyone who is interested in technology; particularly free & open source software.