I like the SQLite railroad diagrams. They are generated with pikchr[1], "a PIC-like markup language for diagrams in technical documentation". See the syntax diagram under examples.
Like many accompanying tools, pikchr comes from the hand of SQLite creater Dr. Richard Hipp (and anyone can guess what that means for the quality of the program).
Now pikchr has become my "goto" tool for professional looking diagrams and sketches of all kinds. With its intuitive syntax grammar it is quick to learn - you should give it a try if not yet done!