AlternativeTo - Crowdsourced software recommendations
alternativeto.net
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.

Alternativeto is not itself FOSS or anything, but it’s nonetheless a handy tool for finding open alternatives for proprietary software & services.

There’s an Open Source filter under Licenses.

I’ve found a lot of good stuff there.