The most straightforward is a font I have used on many 'studs out' mosaics. I have used it to decorate the border on the White Rabbit Mosaic as well as 'sign' many of my other mosaics.
Such a font is detailed by the image below:
It is quite easy since it is based on a simple square grid pattern. This is pretty much the smallest size I use in this fashion since any smaller and one would lose adequate resolution (I once designed a font based on the face of a Rubik's Cube -- 3x3 grid cells -- but one had to use one's imagination on some of the more intricate letters).
A more complex style of lettering uses plates and tiles in a SNOT (Studs Not On Top) fashion. Here is one example:
The cells in that image are rectangles of side proportion 2:5 (that of the edge of a LEGO plate element). I'll leave it to the reader to figure which pieces must be smooth tiles, and which pieces can be consolidated into bricks. I have used white and black pieces to build the first half of the alphabet above (A-M). One must first build a border of stacked bricks and then slide the letters into the encasement. A closer view of the 'A' and 'B' is available.
Of course, this page is hardly complete. Larger fonts can be designed... ones with more intricacy. Lowercase letters may also be formed. If anyone has further examples of fonts they have designed, let me know. I'll be happy to link to your examples.
More organic fonts are sometimes called for as well. One example is the lettering I used in the sculpture of Emily The Strange. I used a number of slopes to create large letters that closely matched the original fonts.