I am currently learning the ins and outs of working in C with Pd, and making my own external objects (externs). This list may currently be short, but it is due to expand rapidly soon!
Cusp~: See relevant posts here and here. A signal rate model of the cusp catastrophe taken from catastrophe theory. Used in introducing complexities to simple control interfaces to enable more skilled, finessed and natural mappings without losing ease of use. Or used however you would like to use catastrophic models in sound synthesis! Maybe wave-shaping? You decide.
Download here. Included in download is extern compiled for 32-bit Mac, source, help patch, read me and makefile. If you are planning on recompiling, note that this extern uses the polynomial root solver in the Gnu Scientific Library, which will need to be installed on the machine and linked in the makefile.
Also, a fun FM/touchpad patch can be found here. It includes a port of the fingerpinger external, that reads data from an apple trackpad (magic or laptop). You’ll need to install that before it works.