Below i list some very usefull but free open source software:
Network Stuff
M0n0wall – Router
PFSense – Router (Based on M0n0wall)
Smoothwall – Router
FreeNAS – NAS Server
Untangle – Webfilter
CloneZilla – Hard Drive/Network Cloning
Nagios – Network Monitoring
WireShark – Protocal Analyzer
Real VNC – Remote Support Tool
FOG – Network Cloning tool
ThinStation – Thin Client
Back-Up / Restoring Software
Zmanda Open Source Backup – Free Open Source Backup/Restore
Telephony Stuff
TrixBox – IP PBX
HUD – Telephone Operator panel
AskoziaPBX – OpenSource IP PBX based on M0n0wall+Astrisk Code
Software
OpenOffice – MS Office alternative
AVG – AntiVirus
InfraRecorder – CD Burning Software
7-Zip – WinZip Alternative
NotePad++ – MS NotePad powerfull Alternative
Thunderbird – outlook alternative
FireFox – Internet Explorer Alternative
Internet/CRM’s
Sugar CRM – Free Web Based CRM
E-Mail Servers
Open-Xchange – MS Exchange alternative
Mail Enable – Free Edition of MailEnables email server
Operating systems
Ubuntu – Windows Alternative
OpenSUSE – Windows Alternative
Operating System Management
WebMin – Web based GUI for system Management