The Linux Terminal Server Project adds thin client support to Linux servers.
Thinstation is a basic and small, yet very powerful, Open Source thin client operating system supporting all major connectivity protocols: Citrix ICA, NoMachine NX, 2X ThinClient, Microsoft Windows terminal services (RDP, via RDesktop), VMWare Horizon View, Cendio ThinLinc, Tarantella, X, telnet, tn5250, VMS terminal and SSH.