Apps with 'Multiple DataBase' feature
DBeaver - Universal Database Manager and SQL Client. DBeaver is free and open source (GPL) universal database tool for developers and database administrators.
Gitea is a community managed fork of Gogs, lightweight code hosting solution written in Go and published under the MIT license.
LINQPad lets you interactively query SQL databases in a modern query language: LINQ. Kiss goodbye to SQL Management Studio!
SQuirreL SQL is an open-source Java SQL Client program that will allow you to view the structure of a JDBC compliant database, browse the data in tables, issue SQL commands etc.
Aqua Data Studio is productivity software for Database Developers, DBAs, and Analysts. It allows you to develop, access, manage, and visually analyze data.
QueryPie is a cross-platform database tool that runs on macOS, Linux, and Windows.
Database design and modeling tool - DeZign for Databases.