You're almost certainly using some of it right now! Most of the visitors to the site browse here using Firefox, an open source project of the Mozilla Foundation; the second runner up is Google Chrome, which is just the Google-branded version of the Chromium browser open source project.
GNU/Linux (operating systems)
Wordpress (blogging tool)
Thunderbird (email client)
Blender (3D graphics)
VLC (media player)
FileZilla (FTP client)
Adium (chat client)
Pidgin (chat client)
Audacity (digital audio editor)
FrostWire (P2P filesharing)
OpenOffice/LibreOffice (office productivity suite)
MediaWiki (wiki software that runs Wikipedia)
phpBB (internet bulletin board)
GIMP (graphics editor)
Notepad++ (text editor)
HandBrake (media transcoder)
Calibre (ebook manager)
LaTeX (document preparation system)
Sage Math (computer algebra system)
...and the list goes on.
Even if you don't think you have any installed, most of the computers that host webpages are running the Apache HTTP Server, an open source project. Proprietary software also reuses chunks of open-source software to accomplish common software tasks without having to write everything from scratch. And that's one of the greatest things about the open software movement: it creates a huge bank of code that anyone is free to draw from, study, and incorporate into their own projects—once something is 'done right', the product is available to everyone, for free, forever. This is in contrast to many proprietary software projects, where corporate policy is to patent every feature and algorithm to keep anyone else from using it for 17 years.
