Have you heard of Polyblend Grout Renew? It's a paint-like product (a "colorant and sealer") made especially for grout. There are lots of tutorials online, including this one and this one and it makes a world of difference! One DIY-er commented that she first tried everything - all kinds of cleaning solutions, even scraping the top layer of grout - and nothing worked. So if you have any grout that is in really rough shape and could use a makeover, this could be a great solution.
It can be a bit tedious...but look at the difference here - it makes the floor seem brand new.
You can even tackle those really dark, extra-wide grout joints! :) Check out one more before and after room here (talk about making a truly dated floor feel current!)