Frequently used tools:

eXtplorer File Manager PHP File Manager
AjaxExplore PHP File Manager
AjaXplorer (renamed to Pydio) Alternative to Dropbox
Xerela Network configuration and change management
mturoute Determine the lowest MTU from host to destination
WinMTR A win32 with GUI version of mtr (http://www.bitwizard.nl/mtr/)
Firewall Builder  GUI firewall management application
DhcpExplorer Discover DHCP servers on your local subnet or LAN
OMD – The Open Monitoring Distribution
Check_mk
EMCO Ping Monitor
PingPlotter
SmokePing