\usepackage{parallel}
...
\begin{Parallel}{<left-width>}{<right-width}
\ParallelLText{left-text}
\ParallelRText{right-text}
\ParallelPar
...
\end{Parallel}
Parallel can get terribly confused with colour changes, in
PDFTeX; the indefatigable Heiko Oberdiek has a patch for this
issue — the pdfcolparallel package, which maintains
separate colour stacks for the columns.
The parcolumns package can (in principle) deal with any
number of columns: the documentation shows its use with three
columns. Usage is rather similar to that of parallel,
though there is of course a “number of columns to specify”:
\usepackage{parcolumns}
...
\begin{parcolumns}[<options>]{3}
\colchunk{<Column 1 text>}
\colchunk{<Column 2 text>}
\colchunk{<Column 3 text>}
\colplacechunks
...
\end{parcolumns}
The <options> can specify the widths of the columns, whether to
place rules between the columns, whether to set the columns sloppy,
etc. Again, there are issues with colours, which are addressed by the
pdfcolparcolumns package.
The ledpar package is distributed with (and integrated with)
the ledmac package. It provides parallel
setting carefully integrated with the needs of a scholarly text,
permitting translation, or notes, or both, to be set in parallel with
the ‘base’ text of the document.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=parallel