If you need a diagram of your MySQL database and you’re on a Mac, generating an ERD diagram is quite easy – and completely free. Sequel Pro can export Graphviz dot files, and then all you need is a few tools to create the diagram.

• Install graphviz from MacPorts via your Terminal:
  sudo port install graphviz
• Install Sequel Pro, run the app, connect to your MySQL server and open the database you’d like to diagram.
• Go to File > Export > Graphviz Dot file, and save the file somewhere convenient.
• Generate an SVG file of your diagram:
  dot -Tsvg your_database.dot > your_database.svg
• You can open SVG files with Opera, Safari, Illustrator, etc, but you can generate a PNG file in a number of ways. You can try installing ImageMagick or libsrvg from MacPorts, or use Illustrator or Inkscape to open and convert the file.
  • ImageMagick: convert your_database.svg your_database.png
  • libsrvg: cat your_database.svg | rsvg-convert -o your_database.png

The result is a basic table-based ERD, but it’s not bad for a few minutes of your time.

Thanks to the geert README for the introduction to this process.