Cleaning Grout……what have you used successfully?

A grout pen.
Take the lid off, and just run it over the grout lines.

I use Domestos Grotbuster. It is amazing.

I have used one of the magic erasers (lots of different brands available) on the grout in the shower. Excellent results and no product needed just water and the eraser.

Bicarbonate of soda mixed with white vinegar and a cheap or old toothbrush.
Half a lemon works too.