On a default Ubuntu installation you'll have postfix configured to act as a standalone mail server, it's very easy to set it up to authenticate to a remote SMTP and send all the emails safely and without the risk of them being rejected as spam.
HylaFax is a powerful open source Fax Server that can be used to share a single FAX line with users on LAN or even via e-mail (integrated with Postfix mail server).
This tutorial will guide you through the basic configuration of a single modem and configuration of fax receiving in PDF via e-mail.
There are also a few GUI clients for Windows, Linux and Mac.
Sometimes you need to give access to your linux server to remote users but you want them to be chrooted on their home (for example if it's a shared hosting webserver). With the latest release of OpenSSH it's very easy to achieve that with just a few edit to the configuration file.
Git is a distributed version control system and can be used as an alternative to SVN or CSV, it's very fast and since it's distributed every clone has the complete project history without having to be tied up to the original server. This howto will go over a basic and fast installation tested on Ubuntu 10.10
OpenVPN is a standard and wide-used software to set-up VPN easily and in a multi-platform enviroment. This guide will walk you trough all the necessary step needed to take a standard linux server and transform it in a VPN server with authentication based on certificates, bridging with the rest of the network and NAT compatibility.
This tutorial will get you up and running with a complete isp-grade mail server.
You'll learn how to install Postfix and Dovecot, storing the user data on a MySQL DB and filter the mail with SpamAssassin, SPFs and other tecniques.
In the guide there are also chapters about server-side Sieve Filtering, Roundcube webmail (complete with password change and filter manager) and log analysis with Awstats for Postfix.
This tutorial has an overview of the basic commands and tool needed to manage a Postfix mail server mixed with a few things that are always good to know when managing a mail server.
Usage of tools like postqueue or postcat is covered and also explanation of all the important directories and files used by dovecot.