Standard Galactic Alphabet
The Standard Galactic Alphabet (SGA) is a writing system used throughout the Commander Keen series. It is a simple substitution cypher, where each letter in the Latin alphabet has been substituted with a different symbol. The SGA can be used to write in different languages; however, in the games the SGA is used to write messages in the English language only.
Besides the 26 letters, SGA uses spaces for its interword separation, and in some occasions a ._. symbol as a full stop. In the Keen 1 level in which most players obtain the pogo stick, a short horizontal dash above a long one appears on either side of the word "POGO", perhaps as quotation marks (though not listed in any of the official translations). No other known punctuation exists. There is also no capitalization, and numerals are also absent. For this reason, numbers are normally spelled out in words.
The SGA was created by Tom Hall. Originally for Keen 1 he drew some graphics for 'Exit' signs which he made look a bit more alien by changing the ordinary Latin letters a bit. After that he added other signs saying "hi" and "this is neat" (near Rayguns), and he ended up creating conversions for the other letters of the Latin alphabet in order for the signs to resemble writings in an alien language.
The SGA can be figured out by looking at the many signs throughout the games. While Keen 1 only has 15 letters displayed anywhere (A, B, C, D, E, H, I, K, L, N, O, S, T, X, and Y), later games have more. Keen 3 is particularly helpful, since it has a relatively large number of signs with both SGA and Latin letters.
There is some debate over the shape of some of the letters, such as the Z. In the reference translation above (which is remarkably similar to the translation found in Keen 3), it is shown as two vertical bars with a horizontal bar connecting them at the top. In Keen 6, the translation image is much smaller so each "bar" appears only one pixel wide/high. This gives the impression it should be a rounded shape, like an upside down U. However because of the increased resolution in the Keen 3 version, one could argue that it is not supposed to be rounded, otherwise it would contain curves like the letters N and O do (or even the top of the letter A.)
There are a few SGA TrueType fonts available, which allow you to write SGA text quite easily. They all have different names, so it is possible to have all the fonts installed at the same time without any conflicts.
The VGA hardware is capable of having text-mode fonts dynamically loaded. These typically persist until the screen mode is changed, although the "Font Editor 3.0" program includes a TSR that ensures the font is reapplied each time the screen mode is changed, keeping the font set as DOS programs are loaded and exited.
There is also a non-SGA Commander Keen related pixel-perfect font called the Commander Keen created by Dean Tersigni which is based on the original font from Commander Keen 4-6 menus. This FON formatted font is available in three different sizes (large, medium and small).
In Other Games
Strife contains a texture which appears to be slightly-modified SGA, most prominently in MAP12: The Temple of the Oracle; as pointed out in the second linked page, they appear to read DAMEX from top to bottom.
The Standard Galactic Alphabet was also incorporated into Minecraft in October 2011, when it became used for Enchantments. The enchantment name translated as a meaningless sentence, and had little to do with the resulting enchantment.