Postscript
This is the language understood by expensive printers, but its also a human-writable language, and its quite easy to make some interesting shapes with it. You can find a nice gentle tutorial on postscript here.
To view postscript programs with the maximum quality, display the postscript files themselves or print them.
MetaFont
Similar to MetaPost (and its ancestor), MetaFont is used to generate high quality bitmapped fonts for use in TeX, rather than postscript diagrams.
MetaPost
This is an extremely fun language. Its used to make diagrams which can then be made into postscript (to be printed or included in documents) with the MetaPost program. A picture is created in MetaPost by writing simple algebra to describe the relationships between points in the image and other variables. In the simplest case the coordinates of each point can be entered directly, but much more interesting things are possible. Once the positions of each point are fixed lines and filled shapes can be drawn.
POV-Ray
I've only just started playing with POV-Ray, but it looks like fun. It can be used to generate high quality 3D pictures.