Everything from operating systems to the most popular applications exist as FOSS. Firefox and Linux are probably the better know FOSS out there. Here is a list of other very useful FOSS applications and their proprietary equivalents:
Proprietary/FOSS
MS Office / OpenOffice.org
Adobe Photoshop / Gimp.org
Adobe Illustrator / Inkscape.net
Skype / Gizmo
MSN Messenger / Pidgin
Microsoft Windows / Ubuntu.com
It must be stated that most FOSS is not quite as user friendly as proprietary equivalents. The installation process of some FOSS is also somewhat more complicated than it is for proprietary equivalents.
On the upside, most FOSS applications are backed by large online communities of developpers and users. There exist wikis, user manuals and intuitive tutorials to help users install and make use of various applications, all for free. Furthermore, even though users have no interest in making the effort to get things to work themsleves, they can pay someone to do it. The upfront cost is likely to be equal if not less than the total cost of purchasing proprietary software.
In the long run, users (or companies) will never have to pay for updates, new versions, license fees, etc. If the upgrading and implementation process of FOSS costs money, well, so does it for proprietary software (when was the last time you set up your computer at work... usually the tech does it... techs cost money).
Not only will individuals and companies save money in the short and long run by adopting FOSS, they will also learn new computer skills using standardized applications which aren't at the mercy of software companies (oh, sorry, we don't support that version anymore, you must purchase the newer one).
For a list of most FOSS projects, visit SourceForge.net.
