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.
There are also an article on how to run both Metafont and MetaPost (the programs). Peter Wilson's Some Experiences in Running Metafont and MetaPost 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 MetaPost page.
Other MetaPost tutorials that have appeared are one in English by André Heck, and one in French (listed here because it's clearly enough written that this author understands it), by Laurent Chéno; both have been recommended for inclusion in the FAQ
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 similar document, written more as a document, and presented in PDF.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=mfptutorials