I'm the developer of several Pd externals. Those are some of them:

fluid~ - soundfont external for Pd and Max/MSP

fluid~ is an external build around the free fluidsynth soundfont synthesizer available on www.fluidsynth.org Soundfonts collect digitally sampled instruments. fluid~ converts Pd or (x)Max to a fullblown orchestra with just a single external.

syncgrain~ synchronous granular synthesis

syncgrain~ 0.2 is a direct port of the SndObject SyncGrain by Dr Victor Lazzarini and implements synchronous granular synthesis.

wurley~, rhodey~ and the other guys - FM example instruments from STK

These are the seven example FM instruments from STK as externals for PD or Max/MSP. Included are: BeeThree, FMVoices, HevyMetl (which doesn't sound like True Metal at all, IMO), PercFlut, Rhodey, TubeBell, Wurley.

All available on the Pure Data repository on Sourceforge