Thin Clients

First part of the R&D // More will be added to this post by THE DOOPER

100% pure Linux - Linux client and Linux server
ltsp.org - Linux Terminal Server Project
FreeNX - FreeNX is a free server implementation of NoMachine.com OSS NX components. NX allows usage of X-connections over real slow links like a Modem or ISDN.

Thin client security gateway in combination with Windows
ThinLinc - security gateway - ssh/vnc IN - ica, rdp, x11 OUT.
eDirectorty support for authentication
Tarantella - security gateway, runs on SLES
support for load ballancing for terminal server.

Client
PXES Universal Linux Thin Client is a micro Linux distribution for thin clients that converts any suitable machine into a versatile thin client within minutes. It can access XDM servers presenting a graphical login screen, Microsoft Terminal Servers, Citrix ICA, and VNC servers, LTSP, or a local X session with simple desktop and window manager. A graphical configuration tool allows parameters such as the server to be set. Local devices can be accessed, including sound, printers, and disks. It boots on a variety of hardware, and does not require NFS. It can be used over low bandwidth WAN and VPN connections, and can also be booted from local media like hard disk and CD-ROMs where network boot cannot be used.

Hardware client (based on Linux)
NeoWare