AlternativeTo lets you find apps and software for Windows, Mac, Linux, iPhone, iPad, Android, Android Tablets, Web Apps, Online, Windows Tablets and more by recommending alternatives to apps you already know.
It’s a great place to find alternatives (including opensource alternatives) to services and software.
There is osalt.com, but it doesn’t seem to be nearly as up to date as alternativeto.
Doesn’t seem to have HTTPS so I can’t browse it.