Mastodon
* An open decentralized social media running on the activitypub protocol.
* Misskey
Another open decentralized social media running on the activitypub protocol.
Other social apps
Coppermine (Alternative link)
* A web gallery that looks a lot like classic deviantart or even sheezy art.
====== Costs Money :$ ======
===== Forums =====
* Xenforo** [$60 per month]
The vast majority of forums on the web use Xenforo. It has many plugins and is highly customizable.