If you are looking after Oracle© databases then you might like to try this free dashboard. It is a simple tool that shows you all your instances and their current availability. It runs on Windows but the databases can be on any platform. See the About page.

The Installation Guides are on the Documents page. The videos show you how to install and configure the dashboard.

DBcafé quick tour video
DBcafe tour (12m 18s)

DBcafé installation videos
DBcafe installation part1 (5m 38s)
DBcafe installation part2 (7m 8s)