Here's another but this one's full height and width of each character is 3 pixels, only capitals and numbers are used: http://en.wikipedia.org/wiki/3x3

This service lets you render your own text in 3x3: http://bikko.org/practice/3x3/index2.php

Image of full typeface in 3x3: http://imgur.com/61375.gif

Interesting. The legibility isn't even comparable to the subpixel font in the OP, however.

This guy's done some very interesting stuff on typophile.

In fairness, in the linked post, only the x-height of each character is 3 pixels. The task the designer of 3x3 set for himself was to create a typeface where each character could only be 3 pixels high and wide at most. That said, the work in the linked post is a very clever and precise exercise in optical illusion. It works wonderfully and is ideal for cases where a design calls for legibility at such a tiny size.