Guide dogs are properly the most well known type of service dog. They help people who are blind or have low vision.
Zeke the Guide Dog 2012
Around the world many different breeds are used. Labradors are common, as are Golden Retrievers. German Shepherds are less common but still make wonderful guide dogs. Other breeds are also trained by programs, or owner trained including crosses of the above.
They wear a special harness designed so the blind person can feel direction through the handle.
Guide dogs are trained to avoid obstacles both on the ground and overhanging at head height. They are also trained to find the curb, doors, counters, and anything else their blind handler may need.
In many countries guide dogs handlers have the right to take their dog wherever the general public is allowed; trains, theaters, restaurants, even hospitals.
Read about the history of guide dogs at the International Guide Dog Federation.