usql - Universal command-line interface for SQL databases
DBeaver - DBeaver - Universal Database Manager and SQL Client.
Open PostgreSQL Monitoring - Oversee and Manage Your PostgreSQL Servers
DataGrip - Tool for SQL and databases
Errbit - The open source error catcher that's Airbrake API compliant
HeidiSQL - HeidiSQL is a powerful and easy client for MySQL, MariaDB, Microsoft SQL Server and PostgreSQL. Open source and entirely free to use.