MIDI Controller is a small program that lets you set MIDI controller values using sliders and buttons in a GTK+ window. The GUI is an XML-based Glade file which can be replaced without rebuilding the program, so you can build your own controller GUIs in Glade, with any number of control widgets in any layout you want.
Beast is a powerful music composition and modular synthesis application. It offers multiple input methods like multitrack, piano roll, and pattern editing and supports skins. On the technical side, it has a wide range of abilities like sequencing, unlimited undo/redo history, real-time synthesis with multiprocessor support, full duplex 32-bit audio rendering, precise timing down to sample granularity, on demand loading of partial wave files, on the fly decoding of various sample formats, aliasing free oscillators, and full Scheme scripting support.