GitLab: the most popular and mature GitHub alternative
Bitbucket: the "third party" of the bunch that's no better than the first
GitTea: the "fourth party" that's actually cool but kinda not quite there yet
Gogs: great, but you need to self-host. GitTea is just a community hosted fork of Gogs
SourceForge: wow, they're still around?
Codeberg: centered around open-source projects only. Managed by a non-profit org
Launchpad: run by Canonical (Ubuntu), has a lot of other features/goals than just hosting code
GitBucket: a self-hostable GitHub clone written in Scala
NotABug: another "liberated" version of Gogs
Radicle: imo, the only other one worth looking at in this list besides GitHub and GitLab. It's unique in that it's build on p2p technologies and is censorship resistant
Would love to see other people's one-liner blurbs on these as well
I can vouch for GitLab. I first heard of it in the self-hosted context. If it goes down, it'll either get the community supporting it (open source), or at the very least, a plethora of "Guide to GitLab alternatives" style-posts.