Here is my compiled list, where you can find lots of open source softwares for Linux,Windows and Mac.Most of these good links easily would have buried under exhaustive Google search results.
- Osalt (My Favorate and lots of good alternatives to commercial ones)
- Distrowatch (Up to date list of Linux distributions and their websites links)
- Sourceforge (Exhaustive list)
- Wikipedia list (Long list)
- Freshmeat (Like Sourceforge)