The aim here is to emulate the Unix nroff, which formats text as best it can for the screen, from the same input as the Unix typesetting program troff.
Converting DVI to plain text is the basis of many of these techniques; sometimes the simple conversion provides a good enough response. Options are:
Ralph Droms provides a txt bundle of things in support of ASCII generation, but it doesn't do a good job with tables and mathematics. An alternative is the screen package.
Another possibility is to use the LaTeX-to-ASCII conversion program, l2a, although this is really more of a de-TeXing program.
The canonical de-TeXing program is detex, which removes all comments and control sequences from its input before writing it to its output. Its original purpose was to prepare input for a dumb spelling checker, and it's only usable for preparing useful ASCII versions of a document in highly restricted circumstances.
Tex2mail is slightly more than a de-TeXer - it's a Perl script that converts TeX files into plain text files, expanding various mathematical symbols (sums, products, integrals, sub/superscripts, fractions, square roots, ...) into "ASCII art" that spreads over multiple lines if necessary. The result is more readable to human beings than the flat-style TeX code.
Another significant possibility is to use one of the HTML-generation solutions, and then to use a browser such as lynx to dump the resulting HTML as plain text.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=toascii