Apart from Knuth's book, there seems to be only one publicly-available tutorial for MetaFont, by Christophe Grandsire (a copy in PDF form may be downloaded). Geoffrey Tobin's MetaFont for Beginners (see using MetaFont) describes how the MetaFont system works and how to avoid some of the potential pitfalls.
Peter Wilson's experience of running both MetaFont and MetaPost (the programs), Some Experiences in Running MetaFont and MetaPost (available on CTAN) offers the benefit of Peter's experience (he has designed a number of “historical” fonts using MetaFont). For MetaFont the article is geared towards testing and installing new MetaFont fonts, while its MetaPost section describes how to use MetaPost illustrations in LaTeX and pdfLaTeX documents, with an emphasis on how to use appropriate fonts for any text or mathematics.
Hans Hagen (of ConTeXt fame) offers a MetaPost tutorial called MetaFun (which admittedly concentrates on the use of MetaPost within ConTeXt). It may be found on his company's "manuals" page.
Another MetaPost tutorial in English is: http://www.tlhiv.org/MetaPost/tutorial/ by Urs Oswald. One in French (listed here because it's clearly enough written that even this author understands it), http://pauillac.inria.fr/~cheno/metapost/metapost.pdf by Laurent Chéno.
A three-part introduction, by Mari Voipio, was published in TUGboat34(1) (Entry-level MetaPost: On the grid), TUGboat34(2) (Entry-level MetaPost: Move it!), and TUGboat34(2) (Entry-level MetaPost: Color).
Vincent Zoonekynd's massive set of example MetaPost files is available on
CTAN; the set includes a
Perl script to convert the
set to html, and the set may be
viewed on the web.
While these examples don't exactly constitute a “tutorial”, they're
most certainly valuable learning material. Urs Oswald presents a
written more as a document, and presented in PDF.