One of my most frequest asks of the CRM Product Development team is cascading lookups. So if I select an account, only show me the contacts assoicated with that account. Or one I just did last week, was only show me the vehicles related a contact. (I wish Philip would quit playing with his Zune and build this into the product's future releases...)
This piece of code is unsupported it looks like from the documentation, but I have had no issues with it at all in my demo enviroment. It is around 300 or so for the code, which does not need any additional client licenses. Pretty slick.
Link to A custom lookup dialog - evaluation download.