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)