I use an old toothbrush and a bathroom cleaner with bleach in it. There are several brands, Tilex is pretty effective if you have good enough ventilation to work with it. What are you trying to clean off?
You can also rent a professional grout machine at most hardware stores. It removes existing grout and replaces with new grout. Very reasonable and works great. Make sure that you measure the width of your grout line so that they can match up the proper machine head and contour angle. You should be able to rework most MH grout lines in one days rental