Some open source applications have restrictions on their use and distribution, but many do not. Unlike public domain software, Open source software does have copyrights.
Examples of open source software include: LINUX, Apache, Firefox, KOffice, Thunderbird, OpenOffice, KOffice, and SquirrelMail.
Sometimes the term "Open Source" is mistakenly used interchangeably with "public domain" software but they are not the same thing.
The key distinction between OS and public domain software is not if the source code is accessible, but if there are any licensing requirements or other restrictions on use, altering the source code, redistribution, or copyrights. If there are, it is Open Source - not public domain software.
Open Source Initiative, a 501(c)(3) California-based nonprofit, offers a very detailed (and legal) definition of open source software and how and who can use it. They also have a great alphabetical listing of companies that offer open source software if you wish to investigate a particular company.
