Well, of course, Mozilla Firefox and Thunderbird are open source. I use them extensively.
Linux Mint is open source and that's my OS. That's open source.
Libre Office
Audacity for editing music.
And at work, SpamAssassin is open source, perl, sendmail, mimedefang.
I like K3B for burning DVD's and ISO images.
You might take a look at
http://www.osalt.com/ for suggestions of other open source titles that are popular.