Appwrite
Open-source backend-as-a-service platform with auth, database, storage, and serverless functions
Free tier, paid from $25/mo
Convex
Reactive backend platform with real-time database, serverless functions, and automatic caching
Free tier, paid from $25/mo per member
Directus
Open-source headless CMS and backend-as-a-service that wraps any SQL database with a REST and GraphQL API
Free self-hosted, cloud from $15/mo
Hasura
Instant GraphQL and REST API engine that connects to existing databases and services
Free tier, paid from $99/mo
PocketBase
Open source backend in a single file
Free and open source