A Mandelbrot Set explorer implemented in Haskell using GTK/OpenGL/libqd, with (currently) only one control (left-click to center and zoom in). Multple render threads use higher precision maths at higher zoom levels.
Available from Hackage:
cabal install gmndl
gmndl +RTS -N -qa -RTS --width=640 --height=480
Future releases (probably around December) will provide a more fully-featured user interface.
libqd provides double-double and quad-double number types for C++, with C and Fortran wrappers. The qd Haskell package is still in its infancy, though it is good enough for gmndl there is still missing functionality - hopefully I'll find time to fix this (though probably not until December). You can get the BSD3-licensed source code repository from hsqd on Gitorious hsqd on code.mathr.co.uk, but I haven't got around to publishing the GPL2-licensed source code repository for gmndl yet.