A great way to transfer files between internal and external users. It's great to replaces FTP servers, stop the abuse of smtp to transfer big files (it's a awful protocol to transfer files, but people use because is simple) and helps contain the information that would otherwise be put on third party (the "cloud") file sharing services. Its amazing that is still almost unknown!
A simple text editor for the desktop.
An iptables firewall script with support for ADSL/DSL modems.