Use Clorox Cleaning Bleach Gel to clean your grout. Just pour it on the grout and leave it for minutes. Scrub with a toothbrush and let sit for another 5 minutes. Then use water and a mop to clean everything up
Make your own homemade grout cleaner mixing up a ratio of cup of baking soda, cup hydrogen peroxide, and 1 teaspoon of liquid dish soap. Shake it all up and watch your grout lines restore to their original condition.
Woolite Foam Carpet Cleaner - Sprays out in foam stream so you can spray right on grout line. Let set several minutes, brush, wipe away. Easiest way ever to clean gross grout! I love Woolite carpet cleaner anyway.