Offer information, graphics, freebies, or programs nobody else does. Many Real Estate websites offer Mortgage calculators. Polls and quizzes are also attractive for visitors, if the results calculated and displayed quickly. If you want visitors to return for your polls and quizzes, though, you have to change them often. Freebies are getting more and more popular.
Graphic designers offer free fonts or clip art.
Many people give away screensavers, wallpaper, and even desktop themes for Windows 95 and 98. But the thing that really makes your visitors come back for more again and again is constantly adding to your website. Adding content is essential for return visitors. Add new products to your store. Add new articles.