Vim 7.3 brings the
colorcolumn option, as detailed very well in other answers.
However, if you don't have version 7.3 for whatever reason, you can still achieve a visual indication that you are exceeding a particular column count using vim's
match functionality (see
:help match for details).
Essentially, the match commands allow you to create persistent highlights for text matching a given regular expression.
:match ColorColumn "\%80v." will highlight text in column 80 with the "ColorColumn" group. You can of course substitute any highlight group, and any column value. If you want a strong visual indication, the expression
"\%>79v.\+" will highlight column 80 and beyond.
\%80v means "match in virtual column 80," and
\%>79v means "match after virtual column 79; see
:help /\%c for more.)
This approach will only highlight when there are actual characters present in the specified columns, however, which makes it visually less consistent than