Siren is typically used in consort with a number of other packages; for more information, see the links below

CSL: the CREATE Signal Library: a C++ framework for building OSC-controllable synthesis/processing servers
   http://create.ucsb.edu/CSL

Loris: a sound analysis/resynthesis package written by Kelly Fitz
   http://sourceforge.net/projects/loris

SuperCollider: an efficient synthesis and processing language very similar to Smalltalk
   http://supercollider.sourceforge.net/

CRAM: the CREATE Real-time Applications Manager: a tool for describing and managing complex distributed real-time software systems
   http://create.ucsb.edu/CRAM

Occam: An OSC-to-MIDI translator
   http://www.mat.ucsb.edu/~c.ramakr/illposed/occam.html

Macco: a MIDI-to-OSC convertor
   http://www.create.ucsb.edu/CSL/Macco.zip

FMAK: the FASTLab Music analysis Kernel (a sibling of CSL)
   http://fastlabinc.com/

libSndFile: http://www.mega-nerd.com/libsndfile

PortMidi: http://www.cs.cmu.edu/~music/portmusic

PortAudio: http://www.portaudio.com

FFTW: the fastest FFT in the west
   http://www.fftw.org