ltsp (Linux Terninal Server Project).LTSP allows you to have one computer to do the heavy lifting for several or many thin clients. It is cheaper to purchase thin clients than full desktops, therefore less computer hardware cost overall. Also you have only one computer to support software wise as you only have to update just one machine instead of many. Ltsp has been around a long time and I think was started by Redhat, but picked up by the community. You can cluster ltsp together and build massive networks, It is also very good for small businesses and small schools such a day care center.
Basic knowledge of linux and networking (tcpip setup and usage). If not get a pro to help.
Internet connection for downloading and installing software.
Computer (min 300mhz, but the faster the better) with two nics (network interface cards). the nic that will be used for the thin clients must be set up as static (256 meg ram plus 64 - 128 meg ram for each thin client. (i'm using 512 meg ram and a 800mhz machine for demo purposes.
Some pxe bootable x86 compatible thin clients (or pxe bootable x86 based diskless pc's) with 128 meg ram or better that are setup for getting dhcp dynamically based ip addresses. The more memory the better and no need to statically assign ip addresses of client systems.
Network cabling (as needed). (100mhz or better).
Network switches (as needed). (100mhz or better).
Note: www.rom-o-matic.org allows you to make cd/usb/floppy boot disks if your computer does not support pxeboot.
Later you can also add support for other architectures such as the intel atom or even the old ppc based macs. If you are using a 64bit server, you will need to load the support for the 32bit architecture to support the older x86 systems.
Original instructable: http://www.instructables.com/id/Using-Thin-Clients-a-quick-way-with-Debian-linux/
Part II LTSP Internet access and some safety net. http://www.instructables.com/id/Another-almost-free-computers-thin-client-set-up-P/
Part III LTSP Maintenance. http://www.instructables.com/id/Part-III-LTSP-Maintenance/
Part IV LTSP Clustering (in progress)
Note: the original version of ltsp (v4) supported the old less than 128 mb thin cleints and computers. Some people say that they run both at the same time, but I have never tried it. Still do have a server with the old version of ltsp on it. Just to keep the old thin clients useful.
Setup a Raspberry Pi as an LTSP client: http://www.instructables.com/id/Raspberry-Pi-Jack-of-all-trades/#step7
Note: You can use an RDP client to access MSWindows servera and Vnc to access Apple based systems. So you are not locked out of access those worlds.