In fact, in the world of professional web development, entire subspecialties exist in which individual developers spend the majority of their work developing the applications and databases that form-based, dynamic web pages require. You'll see the results of such complex specialties at many familiar websites. Anything with e-commerce, such as Amazon.com, requires this kind of complex development to properly process orders, credit card information, and shipping directions.
Portal sites are also very good examples of complex use of forms. Yahoo!, Excite, and even America online all use forms extensively to manage membership, personalization features, and so forth.
Still another excellent example of complex forms and data management comes via online banking. I don't know about you, but I'm sure at a loss these days without instant access to my accounts from anywhere in the world. It really helps a somewhat distracted person such as me stay on top of the practical side of life.
Not using online banking yet? Check and see if your bank offers online banking options. My bank, Wells Fargo, which services the western United States, has an excellent website, http://www.wellsfargo.com/, and is even working toward making the site completely standards compliant and accessible.
Okay, have you had enough of forms? Great! It's time to move on to frames and learn their value in contemporary design. I'll also point out their many flaws and tell you why most people have eliminated frames from site designs altogether.