There is a special knife made to remove grout from between tiles. It has carbide dust on the cutting edge. It does a nice clean job, although it does require quite a bit of elbow grease.
For a power version for grout removal follow this link for the FEIN MultiMaster tool. Or for a DIY version you can get a grout removal bit to go with your Dremel. Some of the rotary tool companies even have a grout removal "guide" attachment.
Very inexpensive Grout knives in one form or another are now commonly available in hardware and tile stores.