I've had great luck with Bar Keeper's Friend. The prior homeowner decided that white grout combined with water that has the highest iron content in the world was a good combination. Even with regular cleaning, it turns a lovely dark rust color, which you can imagine looks lovely with the bluish gray cracked ceramic tiles. I just wet down a small area at a time, dump a little BKF on a scrub brush, and in no time, it's white again. I just rinse it off and move on to the next area. It works on rust and hard water stains, but we don't have any light colored grout on the floor that would get more dirt and dust stains, so I cannot guarantee that it would work on your floors. But it is very inexpensive and I use it for cleaning lots of things around the house.

How about have someone regrout it with dark stuff or at least one of the new grouts that resist getting so grubby?

I think I'd get a pot of black coffee and just try to get it all matching brown and pretend it's supposed to be that way. :P

Grrr...that dirty grout problem. I tried a product called Grout Magic, or Tile Magic, something like that...in a spray can. It cleaned the grout, but it was soon dirty again. If you like the tile, take out the grout, and regrout with an epoxy grout in a darker color. Big nasty job, but might be worth it. Or you can stain the grout. There are some good grout staining products available. This is just one of the grout colorants on the market.

Here is a link that might be useful: Grout Revive

I just tried baking soda and it worked great. Put a liberal amount on the grout, add water to make a paste, scrub it in a bit. Let that sit for a while. I think I did 20-30 minutes. Scrub it well and rinse off with a rag (gritty and messy but not that bad). I think I wiped the tile part down with vinegar after that.

I've read that people also use vinegar with the baking soda. I love that combination but don't remember doing that when scrubbing. They say it works great. I'd try without first because acid is not good for grout.

I've heard that windex and steam effectively cleans grout. I've tried the windex but without the steam since my shark steamer bit the dust. I'm contemplating getting another steam cleaner, but haven't as of yet(can anyone recommend one?). The windex with a grout brush worked well as a spot cleaning method. My kitchen floor is rather new, so haven't had to clean grout on the entire floor. Good luck!..plan to follow this thread for alternate methods that work!

btw, I scrubbed the tile with the baing soda mixture too. (Wiped it with the vinegar afterwards after rinsing).

Hot steam as well as acids are hard on grout and can cause it to break down. So be careful what you put on your grout.

Hey Dallan and fellow members. My name is J, owner of Tile Cleaning firm, I just stumbled upon this great forum and I hope I can help out in some way. Dallan, some things to take into account here is the type of tile you have, if it's man made(ceramic, porcelain) you can use an Acid cleaner. If it's stone or natural tile, you'll need an alkaline cleaner. Now, if it's been a while since you've had a professional clean your tile, then the chances of reviving the grout lines with just a cleaning, are fairly slim. In most cases, when grout has been neglected for some time, those dark or blotchy areas are permanent. Not to mention, if a water based sealer was used, the chances of the grout getting dirty again is fairly great. Now, with that being said, one of the best routes for everyone to go when you've diagnosed your grout lines to be beyond cleaning, is to color seal your grout lines. Color sealing will bring your grout lines back to life as well as seal your grout for 5-10years, possibly more depending on the quality of color sealant you use. Most professionals like myself are privy to quality based color sealers and our customers enjoy the benefits. I would do some research on your local professional tile cleaning firm and see what they may be able to do for you, the process of cleaning with the use of specialized chemicals and high pressure & steam is unparalleled. I hope I was able to shed some light on this topic. I'd love to help as much as possible, so please don't hesitate to ask!

Here's a few youtube video's of what color sealing can do for you:
http://www.youtube.com/watch?v=YacuzbznouI
http://www.youtube.com/watch?v=FhBpzuNeJT4&feature=related

Here is a link that might be useful: Color Sealing before & after pictures.

Thank you for the suggestions! I should probably clarify that we have light colored (almost white) ceramic tile and the grout should be a light gray. Most of it shows as medium brown due to the dirt. I don't like the darker grout lines and would like to restore the light gray grout lines. TilecleaningHouston, would I be able to color a lighter color over a darker one? Also, my parents' grout has never been scrubbed and it looks the same after more than 20 years. Why is this? Did they have a better sealer than our grout does? Do you suggest a sealer I can use after the grout is cleaned? If I do clean it myself, it's not something I want to do every year so if I can buy a few years w/ a sealer that would be best. Thanks again!