These come in several forms: original HYPERCARD client for MACs; new SUPERCARD client for MACs; xcave client for Suns using X, Dom's quickieX-client for any System V compatible X /UNIX system; the CLI (command line interface) client for simple terminal access on UNIX systems.
Various languages depending on the client: HYPERCARD, SUPERCARD, C, C++.
You need something to run TAS/ - really these can always undergo another revision.
None of these are very robust. The Mac clients are slow because they are interpreted. Xcave is quite complex because of its additional support for active office sensing. The biggest problem is that each handles interactions with the servers in slightly different ways. We are working on a solution that will eliminate much of this, but it is just in the works.