The current version of Structure Synth is Version 1.5 (Hinxton).
Windows build (for XP, Vista, and 7):
Zip-archive (portable, just extract and run):
Mac build (Universal binary, Mac OS 10.5 and later):
(Thanks for David Burnett for providing this - for an outline of the build process see this forum post)
For Debian and Ubuntu, there is a 'structure-synth' package. (Thanks to Miriam Ruiz for maintaining this).
For other platforms, you will have to build it yourself. See the build instructions below. The source of the latest release can be found here: StructureSynth-Source-v1.5.0.zip
For the latest changes, it is recommended to pull the source code directly from the SourceForge repository. It can be accessed using SVN (see instructions).