Are you talking about epoxy grout? If so, I have seen it used but not used it myself. Usually used in high hygiene or very heavy traffic areas. Not particularly easy to use and apparantly best if you have help.
Sometimes there are grouts with ad-mixes that change the properties slightly making them more flexible and water repellant. BAL have admix GT1 and Ardex do something called E101.
Yes grumpy is right a 2 part grout is usually epoxy. there are new advancements in epoxy grouts now and most can be washed off with water and an emulsifying pad...great for water proof joints in commercial showers and commercial kitchens but not much call for it in the domestic market....there are many single part flexi grouts about now that are much easier to use and clean off...........dave...
Not all 2 part grouts are epoxy, some are just regular grout mixed with a flexible additive to enhance flexibility, very useful for wide joints on wooden floors etc.