Here is but a partial list of my expertise in open source (and free) software:
Linux (CentOS/RedHat, Debian, and Ubuntu in order of preference) - A free, open source operating system capable of doing most things that Windows servers can do only you don't pay for it.
WebMin/UserMin/VirtualMin - Who says you can't manage your Linux server with a GUI (Graphical User Interface)! And this is a web based GUI so that means you can use it remotely as well.
Apache - One of the most popular web servers on the Internet.
Bind - A very popular Domain Name Server software
Squid - An excellent proxy to ease the burden on your outgoing Internet connection and used in reverse, to ease the burden on overworked web servers.
OpenLDAP - An open source implementation of the Lightweight Directory Access Protocol. This performs functions similar to what Active Directory does for Windows Domains.
MySQL - Probably the most popular open source database available and in use by many companies large and small. I have also built database clusters using the MySQL Cluster Edition and Percona XtraDB Cluster.
GlusterFS - An open source, distributed file system capable of scaling to several petabytes (actually, 72 brontobytes!) and handling thousands of clients.
Samba - Need a server in your office full of Windows PCs? Samba on Linux will make your Windows PCs think they are talking to a Windows server. Connect to file shares or printers on the Linux/Samba server just as though you were connecting to a Windows server.
Munin/Nagios - Powerful baseline and monitoring tools for both Linux AND Windows servers.
Joomla - This site was designed by me in Joomla, one of the world’s most popular open source CMS (Content Management Systems) available.
MailScanner - Have a problem with email or viruses clogging your inbox? This is probably one of the best open source anti-virus/anti-spam packages out there to combat the problem.
pfSense - If you want a firewall on a Linux server, it's hard to beat iptables which is why I turn it on all the Linux servers I administer. However, if you want a firewall at the gateway of your small business, this is one of the most feature-packed and dependable I have worked with.
Perl - A highly capable, feature-rich programming language with over 25 years of development. I have probably written over 200 scripts in Perl over the years to accomplish a vast array of tasks on both Windows and Linux.
VMWare ESXi - I have also implemented both Linux and Windows in a virtual environment using this excellent virtual machine software.
KVM - (Kernel-based Virtual Machine) is a full virtualization solution for Linux on x86 hardware containing virtualization extensions (Intel VT or AMD-V)
XenServer - an enterprise-class, cloud-proven, virtualization platform that delivers all of the critical features needed for any server and datacenter virtualization implementation.
DRBD - Block devices designed as a building block to form high availability (HA) clusters.
Ceph - A distributed object store and file system designed to provide excellent performance, reliability, and scalability.
I have worked with SSO (Single Sign On) between Windows and Linux using several different technologies
Backup and Disaster Recovery - Over the course of my career, I have developed and implemented numerous backup and disaster recovery plans and I can create and test one for you too . . . and many times I can do it without expensive hardware or software. Rarely is there a need to go out and buy an expensive tape drive system and backup software.