More technically known as promotional marketing products or ad specialty items, they're often referred to as tchotchkes (pronounced chotch-keys). And there are no rules to what you choose-just find something that will successfully get your company's name and mission into the heads of customers.
If you own a financial service, a good fit might be pocket-sized solar calculators. Or, if you have a car-repair shop, you could give away tire pressure gauges imprinted with your logo and phone number. A popular choice for Internet-related businesses is mousepads with a logo and URL. Seasonal tchotchkes include fleece vests and beach towels.