Somebody asked this last month and got some good answers. I recommend Barkeeper's Friend, as somebody else noted, but I have also heard oxygen bleach does the trick, as Dan says here:
There is also a link in there to a good article by my Colorado homeboy Kevin, who knows everything and does a lot of Hometalking.
hydrogen peroxide better than bleach. it is basically the same thing used by dentists to whiten teeth
Cream of Tartar works very well in grout!
If the grout is old you may have to paint it.
Paint is the worst thing you could do to grout, IMO
There are grout stains to match your color you can resurface your grout with. But you will have to burnish (take the surface area off)existing grout before you apply stain. Then seal with a higher dollar grout/tile sealer. The less expensive sealers don't seem to stay as long....
I used a steam cleaner and it worked great!
I have heard that white distilled vinegar works well to clean grout. (I have not tried this.)Anyone else heard this?
I used that tooth whitening stuff or baking soda and white vinegar,you sprinkle the baking soda on the counter top and pour vinegar ontop of the baking soda it will fizz up let it sit until the fizzing stops and I use a small brush to scrub the cracks to lift the rest of the dirt and wipe with a rag or sponge.
After you have cleaned it well with bleach,rinse it well and let it dry for 8-24 hr. Then apply new grout, It isn't hard. Once you have done this and let it sit put a sealer on it and it should last for years.
I use an R-Teez Grout Cleaner, it's like a big eraser and restores the grout to its original color. It's made by The StainEraser, you can probably buy it on line. It's hard work and needs to be done about every 6 months but it really works, with no chemicals.