A couple of websites that have helped me greatly to find excellent open source projects:
- osalt.com (open source as alternative): here you will find a good list of open source alternatives, a description of each program and the rating readers have given to each project;
- SourceForge: the home of the majority of open source projects, you will also find statistics of the most downloaded programs and their “pick of the month”.
Both of these links are a great place to start. They will list software for Linux, Windows and MacOS.