Open source anti-virus and/or anti-spam software

An open source web server that can run on Linux, Unix, and Windows

Drafting software by Autodesk

Volunteer computing and desktop Grid computing

An open source Content Management System. http://www.drupal.org

Email client software, such as Thunderbird and Outlook Express

IMAP server packages, such as UW-IMAP, Courier, Dovecot, etc.

LDAP server or client software, such as OpenLDAP

"Open Source/Free Software suite that provides seamless file and print services to SMB/CIFS clients."

SMTP server software, such as Exim, Sendmail, etc.

SQL server software, such as MySQL and PostgreSQL

Caching proxy server software

Secure Sockets Layer software, such as OpenSSL, and software that makes use of it, such as OpenSSH

Webmail software, such as SquirrelMail and Hord IMP

"Open Source implementation of the Windows API on top of X, OpenGL, and Unix"