• SourceHut as an alternative git foundry, https://sr.ht
• Any HedgeDoc deployment , like https://pad.nixnet.services/
• Any jitsi meet deployment, like https://meet.jit.si/
• KeyOxide for online identity verification (using gpg), https://keyoxide.org/
• PlantUML for drawing all kinds of diagrams based on formal text descriptions (online version: https://www.plantuml.com/plantuml/uml/, but mainly just an offline tool). I’d encourage people not to use online services for everything but simply rely on good tools.
• Freesound.org (https//freesound.org) for sound fragments
  • TrekCore.com for Star Trek sounds ;) https://www.trekcore.com/audio/
• Bookwyrm as a fediverse alternative for sites like goodreads (tracking and reviewing books): https://bookwyrm.social/
• Fotostrips.nl if you’re dutch and like comics

And in general I’d wish more people would use RSS (and more sites would prominently offer it) to aggregate things like news.