\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, 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.
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