To use DPS clients, you need to install a DPS server side, which can be either a server extension (a ``DPS/X extension''), or a separate process (referred to, variously, either as a ``DPS/NX agent'' or, rather confusingly, as ``Client-Side DPS'' (CSDPS).
Display Ghostscript (note the capitalisation), or DGS, is a client-side implementation of DPS based on the Ghostscript PostScript interpreter. DGS is still in beta at the time of writing; it does, however, provide a very usable implementation of DPS, although it still has some problems with the semantics of multiple DPS contexts.
DGS is available from the GNUstep download area.
The DPS extension is a much younger project aiming at producing an efficient server-side implementation of DPS. The extension is currently in a state best described as alpha; current versions are known to crash the X server under some circumstances.
The DPS extension is available from the DPS extension site.