FlowCanvas is an interactive Gtkmm/Gnomecanvasmm widget for "boxes and lines" environments (ie modular synths or interactive finite state automata diagrams).
It is used by Ingen and Patchage (among other things).
Screenshots
See Ingen, Pathchage or Machina for some examples of what FlowCanvas can do.
Download
The latest version of FlowCanvas is 0.5.0.
Experimental Debian packages are also available (or can be built from source with make deb)
Documentation
Development
The most recent development version of the code can be checked out of SVN at http://svn.drobilla.net/lad/flowcanvas.
Chances are if you want FlowCanvas you want to check out the complete repository:
svn co http://svn.drobilla.net/lad
Development of FlowCanvas can be tracked here (change history, bug reports, etc).
Bugs
Please file any Patchage bugs you find here