Raul Diaz Poblete created his own softsynth TR-909 using Pd for the sound and Processing for the GUI. I'll let him describe it:

Audio processing of tr-909 emulation is carried out by Pure Data, while its graphical interface is programmed with Processing. Both applications (graphical interface and digital audio processing) are communicated by OSC (Open Sound Control).

As well as audio processing, Pure Data deals with application management: data management, rhythmic patterns loading and state saving (using sssad library by Frank Barknecht). Audio processing is based in Roland TR-909 real samples modified with Pure Data. To use tr-909 emulation you only need to download the application you will find in the lower link, unzip the file, and open the executable file START_TR909.bat. There is no need to install either Pure Data or Processing, only Java Plugin is needed.