Mastodon
Mastodon is one of the biggest decentralized microblogging platforms — an alternative to Twitter.
I have an account at Mastodon.
Mastodon-Twitter crossposter apps exist, but since Twitter is removing free access to the API, many of these will no longer work.
Web Clients
Mastodon has a built-in web client. There are native Android and iOS applications as well.
Elk is a really promising web client that looks very nice. Worth keeping an eye on.