Office.
Open Office http://www.openoffice.org
Staroffice http://www.staroffice.org
Mail.
MTA.
sendmail http://www.sendmail.org
Exim. http://www.exim.org
Courier-MTA http://www.courier-mta.org
Postfix http://www.postfix.org
Qmail http://www.qmail.org
Mailstore.
Courier-IMAP http://www.courier-mta.org/imap/
MUA.
Evolution http://www.gnome.org/projects/evolution/
Horde (a web based email client) http://www.horde.org
Anti-Spam.
Spamassassin http://spamassassin.apache.org
ClamAV http://www.clamav.net
Calendering and Groupware.
Phpgroupware http://www.phpgroupware.org
Openxchange http://www.novell.com/support/products/openexchange/
Opengroupware http://www.opengroupware.org
Web Services.
Browsers.
Firfox http://www.mozilla.com/en-US/firefox/
Web Servers.
Apache http://www.apache.org
Zope http://www.zope.org
Content Managment Systems.
Joomla http://www.joomla.org
Drupal http://drupal.org
Document Managment
Alfresco Document Manager http://www.alfresco.com/
Tutos http://www.tutos.org
Databases.
Mysql http://dev.mysql.com
Postgress http://www.postgresql.org/
Operating Systems
Fedora http://fedora.redhat.com
Debian http://www.debian.org/
CentOS http://www.centos.org/
FreeBSD http://www.freebsd.org
Slackware http://www.slackware.com
Suse http://www.novell.com/linux/
T2 http://www.t2-project.org/
Security.
OpenSSH http://www.openssh.com
Proxy Server.
Squid http://www.squid-cache.org
Firewall
Snort http://www.snort.org
Shorewall Router and Firewall http://www.shorewall.net
IPCop http://www.ipcop.org/
Trustix http://www.trustix.org
pfSense http://www.pfsense.com/
Router
XORP http://www.xorp.org
Vyatta http://www.vyatta.com
Network Monitoring
Nagios http://www.nagios.org/
OpenNMS http://www.opennms.org/
S.M.A.R.T. http://smartmontools.sourceforge.net/
ZenOSS http://www.zenoss.com/
Virutal Private Networking
Openvpn http://openvpn.net/
Remote Access
scp http://www.openssh.com
TightVNC http://www.tightvnc.com/
File Storage.
FreeNAS http://www.freenas.org/
Openfiler http://www.openfiler.com/
NFS http://nfs.sourceforge.net/
Samba http://www.samba.org/
Backup
Amanda http://amanda.zmanda.com/
System Admin.
Webmin http://www.webmin.com/
DNS
Bind http://www.isc.org
Directory Services.
OpenLDAP http://www.openldap.org/
Window Emulations.
Wine http://www.winehq.com/
ReactOS http://www.reactos.org/
Virutulization.
Xen http://www.xensource.com/
Win4lin http://www.win4lin.com/
Frameworks.
Seagull PHP Framework http://www.seagullproject.org/
CakePHP http://www.cakephp.org/
Eclipse http://www.eclipse.org/
ERP
Openbravo http://www.openbravo.com/
Compiere http://www.compiere.org/
Opentaps http://www.opentaps.org