Appointment Setting
We needed a tool that would allow a website visitor to see an appointment online and available, and then register themself for that particular time, thereby removing it from the actual online calendar.  This is a very sophisticated application that can be applied in many ways, and can be customized even further to satisfy any number of specific goals.

Survey system/filtering system/calculation system to query a product database for an Ecommerce application
One of the most inventive, but the most useful, tools we've built, was one that accomodated a client's need to provide their visitors the ability to "search/survey" their products using a WIZARD of sorts.  With it, a user inputted measurements needed to calculate specific variables.  Then, a database was queried for applicable products that would fit the application, both in a potential product category, but also in a product class, as a result of the calculations completed.

AJAX enabled product selection
Some website visitors become frustrated by a product search that results in no findings, so we created an interesting applications that uses AJAX technology and only allows a visitor to filter (using drop-down boxes) under areas where there an eventual search can be completed.  In this kind of application, each drop-down that filters results depends on the others, bringing a whole new complexity with more and more product sets.