Firebase
Spark plan (no payment method): Firestore, Auth and more
Limits: Firestore: 1 GiB stored, 50K reads/day, 20K writes/day; Auth up to 50K MAUs
Free Databases tools
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MongoDB Atlas
Free cluster forever, for learning and exploring
Limits: 512 MB storage, shared RAM and vCPU
Neon
Free serverless Postgres, no credit card, no time limit
Limits: 100 projects, 100 CU-hours/month and 1 GB storage per project, up to 2 CU
Supabase
Free Postgres, auth, storage and edge functions
Limits: 500MB database, 2 active projects, pauses after 1 week inactivity
Upstash
Free Redis database
Limits: 250 MB data, 10 GB bandwidth, 500K commands/month