Two well-reviewed grout cleaners are Hoover 2X FloorMate Tile and Grout Plus and ZeP Grout Cleaner and Whitener. The Hoover product can also be used to clean granite and marble, whereas the ZeP product cannot.
Hoover Tile and Grout Plus is designed for a Hoover FloorMate cleaning device. Some reviewers stated that the Hoover grout cleaner was able to clean grout that was stained for several years. A lot of reviewers also stated that the product cleaned without leaving scum and dirt behind. However, some reviewers required several applications and significant force when scrubbing. There were also some reviewers that complained of a foul order, however, the Hoover grout cleaner got mostly positive reviews with 4.2 out of five stars on Amazon.com.
The ZeP grout cleaner cleans and whitens grout without the use of bleach, but cannot be used on marble, stone, metal or corian. The product also claims to be effective without much scrubbing. Most reviewers said that it worked well on floor grout but not so well on other types of grout, regardless of the color of the grout. Also some reviewers stated that the product was good at brightening the grout but not removing mildew. Overall the product got 4.4 out of five stars on HomeDepot.com.