(Thanks to 'bc' telling me what it's technically called), it's called a Dremel. Here is a link that may be of use. Removing Grout.
It does take some skill to use. If you're not careful, you'll nick the he-2-sticks out of the surrounding tile! Not only will you be removing grout but you may have to remove damaged tile too! You can find this product by searching the WEB for dremel tool. Just get the right bit(s).