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 quickest way to find out if you like the tool is to download it and run it. The installation can be done in a few minutes if you just press enter on each question to accept the defaults. You can download the complete version if you feel like developing it further, or making something similar but better.
DBcafé quick tour video
DBcafe tour (12m 18s)