Ellipses are commonly required, and LaTeX natively supplies a fair
range (\dots, \cdots, \vdots and \ddots). By using
the graphics package, one can change the slope of the
\ddots command, as in
$ ... \reflectbox{$\ddots$} ... $
While this works, it is not a recommended way of achieving the desired
result (see below). Moreover, LaTeX's range is not adequate to
everyone's requirements, and at least three packages provide
extensions to the set.
The AMSLaTeX bundle provides a range of "semantically-named"
ellipses, for use in different situations: \dotsb for use between
pairs of binary operators, \dotsc for use between pairs of commas,
and so on.
The yhmath package defines an \adots command, which is
the analogue of \ddots, sloping forwards rather than backwards.
The yhmath package comes with a rather interesting font that
extends the standard cmex; details are in the documentation.
The disadvantage of this setup is, that although \adots is merely
a macro, the package tries to load its own font and produces a
"missing font" substitution warning message if you haven't installed
the font.
The mathdots package (besides fixing up the behaviour of
(La)TeX \ddots and \vdots when the font size changes)
provides an "inverse diagonal" ellipsis \iddots (doing the same
job as yhmath's \adots, but better).
Documentation of yhmath appears, processed, in the distribution (thus saving you the bother of installing the package before being able to read the documentation). Documentation of mathdots appears at the end the package file itself.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=mathlips