hl-column

Provides a local minor mode (toggled by M-x hl-column-mode) and a
global minor mode (toggled by M-x global-hl-column-mode) to
highlight, on a suitable terminal, the line on which point is.

You probably don't really want to use the global mode; if the
cursor is difficult to spot, try changing its color, relying on
`blink-cursor-mode' or both.

An overlay is used.  In the non-sticky cases, this overlay is
active only on the selected window.  A hook is added to
`post-command-hook' to activate the overlay and move it to the line
about point.