Google Cloud PostgreSQL - Fully-managed database service
SQL Police Department - Learn SQL while solving crimes! Climb the ranks of SQL PD.
Supabase - An open source Firebase alternative
Slack SQL - Execute SQL queries inside of Slack
SQL Play - Run SQL in your phone
pganalyze Index Advisor for Postgres - The index advisor makes recommendations for creating the best indexes for your Postgres queries, based on the schema and table statistics information.