Well-thought-out trade-show giveaways do work. Irrelevant, unimaginative freebies do not. And beware of catering to giveaway grabbers who will dart into your booth, grab a freebie and dart back to the aisle without so much as making eye contact with your staff. Give out freebies to booth visitors individually rather than creating a free-for-all. Or make a game out of the giveaway transaction by having visitors answer a basic question like, "What is the major benefit of our product?" Takers will at least walk away with a message about your product in addition to the clever memento.
Effective giveaways provoke questions and give visitors a reason to pause long enough to learn something about your company. It's also a good idea to develop a theme for your booth and integrate the giveaway into the theme. You don't have to spend a lot of money to devise a great giveaway.