Orthogram is a command line program and Python library that lets you draw block diagrams. It reads a YAML file and produces a PNG file like this one:
Orthogram does not aim to be a fully-featured graph layout solution. It offers just a single layout: grid. You have to arrange the blocks manually in the grid and they stay fixed in place; the program will not attempt to move them around trying to optimize any aspect of the diagram. Styling is also rather basic at the moment. It does however try to do a decent job arranging the connections around the blocks to produce a tidy, readable drawing.
When used as a command line tool, Orthogram reads a diagram definition file and produces a PNG image file. The definition file is written in YAML.
The source code is hosted on Github.