Be nice to this fool^H^H^H^Hgentleman,
he's visiting from corporate HQ.
reads roughly as "Be nice to this fool, er, gentleman…". This comes from the fact that the digraph ^H is often used as a print representation for a backspace. It parallels (and may have been influenced by) the ironic use of `slashouts' in science-fiction fanzines.
A related habit uses editor commands to signify corrections to previous text. This custom is fading as more mailers get good editing capabilities, but one occasionally still sees things like this:
I've seen that term used on alt.foobar often.
Send it to Erik for the File.
Oops…s/Erik/Eric/.
The s/Erik/Eric/ says "change Erik to Eric in the preceding". This syntax is borrowed from the Unix editing tools `ed' and `sed', but is widely recognized by non-Unix hackers as well.
In a formula, `*' signifies multiplication but two asterisks in a row are a shorthand for exponentiation (this derives from FORTRAN). Thus, one might write 2 ** 8 = 256.
Another notation for exponentiation one sees more frequently uses the caret (^, ASCII 1011110); one might write instead `2^8 = 256'. This goes all the way back to Algol-60, which used the archaic ASCII `up-arrow' that later became the caret; this was picked up by Kemeny and Kurtz's original BASIC, which in turn influenced the design of the `bc(1)' and `dc(1)' Unix tools, which have probably done most to reinforce the convention on Usenet. The notation is mildly confusing to C programmers, because `^' means bitwise exclusive-or in C. Despite this, it was favored 3:1 over ** in a late-1990 snapshot of Usenet. It is used consistently in this lexicon.
In on-line exchanges, hackers tend to use decimal forms or improper fractions (`3.5' or `7/2') rather than `typewriter style' mixed fractions (`3-1/2'). The major motive here is probably that the former are more readable in a monospaced font, together with a desire to avoid the risk that the latter might be read as `three minus one-half'. The decimal form is definitely preferred for fractions with a terminating decimal representation; there may be some cultural influence here from the high status of scientific notation.
Another on-line convention, used especially for very large or very small numbers, is taken from C (which derived it from FORTRAN). This is a form of `scientific notation' using `e' to replace `*10^'; for example, one year is about 3e7 seconds long.
The tilde (~) is commonly used in a quantifying sense of `approximately'; that is, `~50' means `about fifty'.