AlternativeTo is a great site for finding alternative software to basically anything. You can search individual apps and see their alternatives, or just browse popular applications in generic categories.
The recommendations are based on user reports. Just create an account and you can post your own recommendations, and help steering people towards good software!
While the site itself is unfortunately not opensource, it does offer great tools for discovering open alternatives to proprietary software. There is an open source tag that you can use to filter the results.