Glen Discovery Ltd Computer Services supports Linux-based servers and recommends Centos 5. Centos is an acronym for Community Enterprise Operating System. RedHat Linux used to be downloadable for free, with chargeable technical support. RedHat then moved to a chargeable licensing structure with pricing comparable to Microsoft server operating systems. The licensing model to support this can be a time-consuming irritation. You have to pay an annual license fee in order to obtain software updates. Centos is, in effect, the current RedHat Enterprise Linux without all the licensing bureaucracy and free access to software updates. You can find more at http://www.centos.org/ Donations are encouraged, but not mandatory. Centos is fairly straightforward to install. In our experience Linux is a more reliable operating system than Microsoft
If preferred, I can support other Linux distros, such as RedHat Enterprise Linux, Ubuntu, Suse Linux.
Samba http://www.samba.org/ provides a domain controller and file server capability for networked windows PCs. For optimum performance, these should be XP-Pro or Vista-Pro or Windows-7 PC. Irrespective of the Windows version, the Professional license is required for full domain membership.
Apache is a part of the Centos Linux distribution. Apache web-servers are the most widespread across the internet. See http://www.apache.org/ A basic internal apache web-server is straightforward to configure and will allow your staff to develop applications without the need to upload to external servers.