Dogs

Dogs are the most common of all pets and are useful in many ways.  They can be companions to the lonely and/or elderly.  Dogs are good alarm systems.  They can be trained to hunt, lead the blind, and sniff out some cancers and diseases.  They help those with depression, or other mental and physical handicaps.

If you're getting a dog, make sure you have time for him.  Dogs require you to take time every day for play, and (if you live in town) walks, and they need to go outdoors to "do their business".   Also, take note of how much space you have.  You don't want to put a Great Dane in a space just large enough for a Chihuahua to run.  So pay attention to this detail.  Puppies are small, but have some idea what size your puppy will be when he is grown. 

Speaking of puppies, they do require much more time and attention than an adult dog.  Plus you generally have to potty train them.  The advantage of having a dog grow up around you and your family, however, can greatly outweight the disadvantages of a puppy that's not trained yet (if you have the time and patience).

Different breeds have different attitudes about things.  Make sure the dog you get is the right one for your needs, and not just a "but he was so cute" ordeal.  Some dogs are more family oriented than others, so do your research and then make a decision based on facts.  That way you avoid the impulse puppy that doesn't really work for you.