Although not required, it is customary to give a gift to the boy or girl who is celebrating their bar or bat mitzvah. Bringing a gift is a great way to help the family celebrate the occasion, and of course, is very much appreciated by the child!
Gifts should not be brought to the synagogue service but rather to the reception/party following the service. There are no "traditional" gifts. Rather, when looking for a gift keep the interests of the child in mind and shop as you would for any other 13-year-old boy or girl. If you don't know the child well you can always ask the parents for help. Usually Judaic gifts like a kiddush cup, tallis or tefillin are given by the parents, grandparents or other people who are close to the family, so it's generally a good idea not to purchase those items as gifts.
Money is often given in multiples of 18, which represents both good luck and the Jewish symbol of Chai, which means "life." Other gift ideas include: iTunes gift cards, iPods, digital cameras, a DVD collection of the child's favorite TV show, video games (ask what game system the child has), clothing store gift cards, Amazon.com gift cards, a Kindle or even a nice purse. If you don't know the child well ask his or her parents for advice. They are always happy to help!