Vtk [L1 ] is a powerful engine for scientific visualization. It's open source.
It has used Tcl/Tk as an integration platform from nearly the beginning [L2 ], [L3 ]. It's also tightly integrated with Java and C++.
Its test suite, Dart, automates nightly regression testing and puts status displays on the Web [L4 ]. It, too, is open source and is discussed further on the testing page of the Wiki.