Recently Heroku announced the end of the free plans and this must have taken quite a lot of people by surprise and made you think "What alternative...
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Fly.io is the number one here. Especially for dynamic server rendered apps and APIs. It offers the best free tier in this list. I'm literally running respectable small to medium apps there, all for free.
Of course if you are building Next.js apps or other Jamstack flavors, Vercel might be the better choice. I use it for those as well.