monitoring

Nagios

Nagios is a system and network monitoring application.

Some of the many features of Nagios® include:

Monitoring of network services (SMTP, POP3, HTTP, NNTP, PING, etc.)
Monitoring of host resources (processor load, disk usage, etc.)
Simple plugin design that allows users to easily develop their own service checks
Parallelized service checks
Ability to define network host hierarchy using "parent" hosts, allowing detection of and distinction between hosts that are down and those that are unreachable

SquidGuard

SquidGuard is a URL redirector used to use blacklists with the proxysoftware Squid. There are two big advantages to squidguard: it is fast and it is free.
NCurses Disk Usage

A disk usage analyzer with an ncurses interface, aimed to be run on a remote server where you don't have an entire gaphical setup, but have to do with a simple SSH connection. ncdu aims to be fast, simple and easy to use, and should be able to run in any minimal POSIX-like environment with ncurses installed.
WPKG

WPKG is an automated software deployment, upgrade and removal program for Windows.

It can be used to push/pull software packages, such as Service Packs, hotfixes, or program installations from a central server (for example, Samba or Active Directory) to a number of workstations.

It can run as a service to install software in the background (silent install), without user interaction.

It can install MSI, InstallShield, PackagefortheWeb, Inno Setup, Nullsoft, other software installers or .exe packages, .bat and .cmd scripts and similar: no more repackaging to perform software installation.

WPKG is open source software.
opsi

opsi (Open PC Server Integration) the Open Source client management system
Vagrant

Vagrant is a tool for building complete development environments. With an easy-to-use workflow and focus on automation, Vagrant lowers development environment setup time, increases development/production parity, and makes the "works on my machine" excuse a relic of the past.