Let's do em all!:
- GitHub: most mature/reliable
- GitLab: the most popular and mature GitHub alternative. Generally seen as a more ethical alternative since it's not owned by MS and is open-sourced, but is still criticized for it's open-core business model
- 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. Worth keeping an eye because it's the most likely to integrate with ActivityPub soon
- 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, one of the most interesting alternatives to look at. It's unique in that it's build on p2p technologies. Unfortunately, it seems quite coupled with many projects in the web3 space
- Pagure: RedHat developed git forge that can be selfhosted
- Phorge: community fork of Facebook’s internal Phabricator forge tool which was deprecated in 2011 but got a lot of things right that GitHub is often criticized for
- Heptapod: Gitlab modified to work with Mercurial
- Fossil: self-contained small team collaboration tool doing its own thing entirely
- Kallithea: git and hg web frontend with code review functionality (community fork of Rhode code)
- RhodeCode: git and hg frontend (original codebase where Kallithea forked off)
- Sourcehut: email centric git frontend
Would love to see other people's one-liner blurbs on these as well
EDIT: added additional alternatives and comments (thanks @poVoq@slrpnk.net especially)