Grout saws come in wide range of quality and prices ($3.50 to almost $20). Common complaints about the less expensive models include blades bending or breaking and handles that are uncomfortable or come off. Marshaltown, a well known manufacturer of masonry tools offers a high qualtiy model that has a sturdy two-inch blade and resilient handle with finger guard. Learn more by clicking here.
If your tile grout has gotten to the point where, despite cleaning after cleaning, it still looks stained, dirty or discolored, then maybe it's time to go a bit deeper. By removing the top layer of grout and replacing with new material, the joints will look like they did when the countertop was new. One tool you'll need to accomplish this is the grout saw. Actually, it's not a saw at all, but more of an abrasive scraper. One note of caution when using this tool -- move slowly and deliberately to avoid having the tool slip out of the grout joint and scratch the face of the tile.